XDH

UniProt ID: P47989
Organism: Homo sapiens
Review Status: INITIALIZED
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Gene Description

XDH encodes xanthine dehydrogenase/oxidase (xanthine oxidoreductase, XOR), the molybdo-flavoenzyme that catalyses the last two steps of purine catabolism: the oxidation of hypoxanthine to xanthine and of xanthine to urate (uric acid). It is a large (~146 kDa subunit) cytosolic homodimer in which each subunit carries a molybdopterin (Mo-co) active-site center, one FAD, and two [2Fe-2S] clusters that shuttle electrons from the molybdenum center to the flavin. The enzyme exists as an NAD+-dependent dehydrogenase (XDH) form that reduces NAD+ to NADH, and can be converted to an O2-dependent oxidase (XO) form — reversibly through oxidation of sulfhydryl groups or irreversibly by proteolysis. The XO form reduces molecular oxygen to superoxide and hydrogen peroxide, making XDH a physiological source of reactive oxygen species. XDH is predominantly cytosolic and is highly expressed in liver and intestine; it is also secreted and detectable in milk/colostrum. Loss-of-function mutations cause xanthinuria type I, characterized by xanthine stones and hypouricemia, and XDH is the pharmacological target of the anti-gout drugs allopurinol, febuxostat and topiroxostat.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0005576 extracellular region
IBA
GO_REF:0000033 		KEEP AS NON CORE
Summary: Phylogenetically-inferred extracellular localization. XDH is a predominantly cytosolic enzyme but is genuinely secreted and detectable in milk/colostrum, so an extracellular pool exists; however, "is_active_in extracellular region" overstates this as a site of catalytic activity.
Reason: XDH is secreted and found in milk (colostrum proteomics), so an extracellular pool is real, but this is a minor, non-core location relative to the enzyme's cytosolic purine-catabolic function.
Supporting Evidence:
PMID:16502470
Human colostrum: identification of minor proteins in the aqueous phase by proteomics.
GO:0009115 xanthine catabolic process
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetic inference that XDH is involved in xanthine catabolism. This is the canonical biological role of the enzyme (xanthine -> urate), independently supported by human experimental data.
Reason: Core biological process; the IBA agrees with human IDA evidence that XDH catalyses the xanthine oxidation step of purine degradation.
Supporting Evidence:
PMID:8670112
To study the expression of human xanthine dehydrogenase/oxidase (hXDH/XO), we
GO:0004854 xanthine dehydrogenase activity
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetically-inferred xanthine dehydrogenase (NAD+-dependent) activity, the defining molecular function of the XDH form of the enzyme.
Reason: Core molecular function, concordant with direct human enzymatic assays and the EC 1.17.1.4 catalytic activity annotated by UniProt.
Supporting Evidence:
PMID:8670112
The COS cells expressed the enzyme predominantly (89.8 +/- 0.3%) in the dehydrogenase form.
GO:0004854 xanthine dehydrogenase activity
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated (RHEA/EC 1.17.1.4-based) assignment of xanthine dehydrogenase activity. Duplicates the experimentally supported core molecular function.
Reason: Correct core molecular function; the EC 1.17.1.4 / RHEA:16669 mapping matches the human catalytic activity.
Supporting Evidence:
PMID:8670112
Significant XDH/XO enzyme activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of transfected COS cells
GO:0005506 iron ion binding
IEA
GO_REF:0000002 		ACCEPT
Summary: InterPro-based iron ion binding. XDH binds iron as part of its two [2Fe-2S] clusters, so this is correct but generic relative to the specific cluster-binding term.
Reason: Correct: iron is a constituent of the [2Fe-2S] clusters directly demonstrated in the crystal structure. It is a broad parent of the more specific 2 iron, 2 sulfur cluster binding term but is not incorrect.
Supporting Evidence:
PMID:17301077
Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition
GO:0005576 extracellular region
IEA
GO_REF:0000044 		KEEP AS NON CORE
Summary: UniProt Subcellular Location keyword mapping (Secreted) to extracellular region. Consistent with the secreted milk pool.
Reason: Extracellular localization is real (secreted, found in milk) but non-core relative to the cytosolic catalytic function.
Supporting Evidence:
PMID:16502470
This is the first comprehensive proteomic
GO:0005737 cytoplasm
IEA
GO_REF:0000044 		KEEP AS NON CORE
Summary: UniProt Subcellular Location mapping to cytoplasm. Correct but a broad parent of the more specific and experimentally supported cytosol term.
Reason: Correct localization but generic; the specific cytosol annotation (with IDA evidence) is the informative core location.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0005777 peroxisome
IEA
GO_REF:0000044 		MARK AS OVER ANNOTATED
Summary: UniProt Subcellular Location keyword mapping to peroxisome (annotated in UniProt by similarity). Direct experimental immunoelectron microscopy of hepatocytes found XO over the cytosol and specifically NOT over peroxisomes.
Reason: Peroxisomal localization is contradicted by high-resolution immunoEM, which showed no XO labeling over peroxisomes; the human XDH is overwhelmingly cytosolic. The peroxisome assignment is a weak by-similarity keyword mapping.
Supporting Evidence:
PMID:1619276
A few gold particles were found over the mitochondrial matrix, but not over the endoplasmic reticulum, Golgi apparatus, lysosomes, or peroxisomes, including their crystalloid core.
GO:0016491 oxidoreductase activity
IEA
GO_REF:0000120 		KEEP AS NON CORE
Summary: Automated assignment of the generic oxidoreductase activity parent term. Correct but uninformative given the specific xanthine dehydrogenase/oxidase activities.
Reason: XDH is an oxidoreductase, but this high-level term is subsumed by the specific xanthine dehydrogenase and xanthine oxidase activity annotations.
Supporting Evidence:
PMID:8670112
Significant XDH/XO enzyme activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of transfected COS cells
GO:0043546 molybdopterin cofactor binding
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated assignment of molybdopterin cofactor binding, matching the molybdenum-molybdopterin (Mo-co) active center directly observed in the human XOR crystal structure.
Reason: Core cofactor-binding function; the Mo-molybdopterin cofactor is directly demonstrated in the human structure and is essential for catalysis.
Supporting Evidence:
PMID:17301077
The overall molybdopterin domain structure of this mutant closely resembles that of bovine milk XOR
GO:0046872 metal ion binding
IEA
GO_REF:0000002 		KEEP AS NON CORE
Summary: InterPro-based generic metal ion binding. Correct (XDH binds molybdenum and iron) but a broad parent of the specific cofactor-binding terms.
Reason: Correct but uninformative generic term; the specific molybdopterin and 2Fe-2S cluster binding annotations capture the actual metal cofactors.
Supporting Evidence:
PMID:17301077
Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition
GO:0050660 flavin adenine dinucleotide binding
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated assignment of FAD binding, matching the FAD cofactor directly observed in the human XOR structure.
Reason: Core cofactor-binding function; FAD is one of the three prosthetic groups required for electron transfer and is directly demonstrated in the structure.
Supporting Evidence:
PMID:17301077
Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition
GO:0051536 iron-sulfur cluster binding
IEA
GO_REF:0000002 		KEEP AS NON CORE
Summary: InterPro-based iron-sulfur cluster binding. Correct but a parent of the specific 2 iron, 2 sulfur cluster binding term supported by the structure.
Reason: Correct but generic; the specific 2Fe-2S cluster binding annotation (IDA) is the informative term.
Supporting Evidence:
PMID:17301077
Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition
GO:0051537 2 iron, 2 sulfur cluster binding
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated assignment of [2Fe-2S] cluster binding, matching the two [2Fe-2S] centers directly observed in the human XOR structure.
Reason: Core cofactor-binding function; XDH binds two [2Fe-2S] clusters that relay electrons from the Mo center to FAD.
Supporting Evidence:
PMID:17301077
Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition
GO:0071949 FAD binding
IEA
GO_REF:0000002 		ACCEPT
Summary: InterPro-based FAD binding, a sibling/near-synonym of the flavin adenine dinucleotide binding term. Correct and supported by the structure.
Reason: Correct cofactor-binding function; FAD is a directly observed prosthetic group of XDH.
Supporting Evidence:
PMID:17301077
Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition
GO:0005515 protein binding
IPI
PMID:32296183
A reference map of the human binary protein interactome. 		MARK AS OVER ANNOTATED
Summary: Bare "protein binding" from a high-throughput binary interactome screen (HuRI; interactor GRIP1 isoform). The term is uninformative and does not describe a specific molecular function of XDH.
Reason: Per curation guidelines, bare protein binding provides no functional insight and derives from a systematic Y2H screen rather than a characterized XDH interaction; retained (not removed) but flagged as uninformative.
Supporting Evidence:
PMID:32296183
we present a human 'all-by-all' reference interactome map of human binary protein interactions, or 'HuRI'.
GO:0004855 xanthine oxidase activity
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated (RHEA:21132 / EC 1.17.3.2-based) assignment of xanthine oxidase activity, the O2-dependent form of the enzyme. Duplicates the experimentally supported core molecular function.
Reason: Correct core molecular function; XDH can operate as an O2-dependent oxidase producing urate and H2O2, directly demonstrated in human enzyme assays.
Supporting Evidence:
PMID:8670112
Cloning and expression in vitro of human xanthine dehydrogenase/oxidase.
GO:0005829 cytosol
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated assignment of cytosolic localization, concordant with the experimentally established primary cytosolic site of XDH activity.
Reason: Core localization; XDH is predominantly cytosolic where it performs purine catabolism, directly supported by immunoEM.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0006147 guanine catabolic process
IEA
GO_REF:0000107 		MARK AS OVER ANNOTATED
Summary: Ensembl orthology transfer of guanine catabolic process. XDH does not act on guanine itself (guanine is deaminated to xanthine by guanine deaminase); XDH contributes only the downstream xanthine oxidation step.
Reason: Whole-pathway BP transferred by orthology; XDH's molecular role is limited to the terminal xanthine -> urate step, not to catabolism of guanine as a substrate.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006148 inosine catabolic process
IEA
GO_REF:0000120 		MARK AS OVER ANNOTATED
Summary: Automated assignment of inosine catabolic process. XDH does not act on inosine (a nucleoside); it acts downstream on hypoxanthine/xanthine.
Reason: Pathway-context over-annotation; XDH participates only in the terminal purine-base oxidation steps of the pathway, not in inosine catabolism per se.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006149 deoxyinosine catabolic process
IEA
GO_REF:0000120 		MARK AS OVER ANNOTATED
Summary: Automated assignment of deoxyinosine catabolic process; XDH acts only on the downstream free base hypoxanthine, not on deoxyinosine.
Reason: Pathway-context over-annotation; XDH's molecular activity is xanthine/ hypoxanthine oxidation, not deoxyinosine catabolism.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006154 adenosine catabolic process
IEA
GO_REF:0000107 		MARK AS OVER ANNOTATED
Summary: Ensembl orthology transfer of adenosine catabolic process. XDH does not act on adenosine; the pathway reaches XDH only after deamination/dephosphorylation to hypoxanthine.
Reason: Whole-pathway BP over-annotation; XDH contributes only the terminal xanthine oxidation step, not adenosine catabolism.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006157 deoxyadenosine catabolic process
IEA
GO_REF:0000107 		MARK AS OVER ANNOTATED
Summary: Ensembl orthology transfer of deoxyadenosine catabolic process; XDH acts only on the free base hypoxanthine downstream.
Reason: Whole-pathway BP over-annotation; XDH's role is limited to the terminal purine-base oxidation.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006161 deoxyguanosine catabolic process
IEA
GO_REF:0000107 		MARK AS OVER ANNOTATED
Summary: Ensembl orthology transfer of deoxyguanosine catabolic process; XDH does not act on deoxyguanosine.
Reason: Whole-pathway BP over-annotation; XDH acts only at the terminal xanthine -> urate step.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006196 AMP catabolic process
IEA
GO_REF:0000120 		MARK AS OVER ANNOTATED
Summary: Automated assignment of AMP catabolic process; XDH does not act on AMP but on the downstream free base hypoxanthine.
Reason: Whole-pathway BP over-annotation; XDH's molecular activity is xanthine/ hypoxanthine oxidation, downstream of AMP degradation.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006204 IMP catabolic process
IEA
GO_REF:0000120 		MARK AS OVER ANNOTATED
Summary: Automated assignment of IMP catabolic process; XDH acts only on the free base hypoxanthine downstream of IMP breakdown.
Reason: Whole-pathway BP over-annotation; XDH participates only in the terminal purine-base oxidation steps.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0009114 hypoxanthine catabolic process
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated assignment of hypoxanthine catabolic process. Hypoxanthine is a direct XDH substrate (oxidized to xanthine), so this is a genuine biological process of the enzyme.
Reason: Core biological process; XDH directly oxidizes hypoxanthine to xanthine, the first of its two purine-catabolic steps.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0009115 xanthine catabolic process
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated assignment of xanthine catabolic process, the canonical biological role of XDH (xanthine -> urate). Duplicates the experimentally supported core BP.
Reason: Core biological process directly supported by human enzymatic evidence.
Supporting Evidence:
PMID:8670112
To study the expression of human xanthine dehydrogenase/oxidase (hXDH/XO), we
GO:0016529 sarcoplasmic reticulum
IEA
GO_REF:0000107 		MARK AS OVER ANNOTATED
Summary: Ensembl orthology transfer of sarcoplasmic reticulum localization. XDH is a soluble cytosolic/secreted enzyme; direct immunoEM found it in the cytosol and not on organelles.
Reason: Weak orthology-transferred localization inconsistent with the established cytosolic distribution of XDH; not a canonical XDH compartment.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0046038 GMP catabolic process
IEA
GO_REF:0000107 		MARK AS OVER ANNOTATED
Summary: Ensembl orthology transfer of GMP catabolic process; XDH does not act on GMP but on the downstream free base xanthine.
Reason: Whole-pathway BP over-annotation; XDH's role is the terminal xanthine -> urate step, not GMP catabolism.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0046055 dGMP catabolic process
IEA
GO_REF:0000107 		MARK AS OVER ANNOTATED
Summary: Ensembl orthology transfer of dGMP catabolic process; XDH does not act on dGMP.
Reason: Whole-pathway BP over-annotation; XDH acts only at the terminal purine-base oxidation steps.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0046059 dAMP catabolic process
IEA
GO_REF:0000107 		MARK AS OVER ANNOTATED
Summary: Ensembl orthology transfer of dAMP catabolic process; XDH does not act on dAMP.
Reason: Whole-pathway BP over-annotation; XDH contributes only the terminal xanthine/ hypoxanthine oxidation step.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0070674 hypoxanthine dehydrogenase activity
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated assignment of hypoxanthine dehydrogenase activity, the NAD+-dependent oxidation of hypoxanthine to xanthine. This is a genuine sub-activity of XDH and is directly supported by human IDA elsewhere in the record.
Reason: Correct molecular function; XDH oxidizes hypoxanthine to xanthine in an NAD+-dependent manner, a component of its dehydrogenase activity.
Supporting Evidence:
PMID:17301077
mutation of two amino acid residues in the active site of human XOR for purine substrates results in conversion of the substrate preference to AO type
GO:0070675 hypoxanthine oxidase activity
IEA
GO_REF:0000107 		KEEP AS NON CORE
Summary: Ensembl orthology transfer of hypoxanthine oxidase activity (O2-dependent oxidation of hypoxanthine). A genuine sub-activity of the XO form of the enzyme.
Reason: Correct sub-activity of XDH's oxidase form, subsumed by the canonical xanthine oxidase activity; retained as a valid but non-core molecular function.
Supporting Evidence:
PMID:17301077
mutation of two amino acid residues in the active site of human XOR for purine substrates results in conversion of the substrate preference to AO type
GO:0005829 cytosol
TAS
Reactome:R-HSA-74247 		ACCEPT
Summary: Reactome-asserted cytosolic localization associated with the reaction "XDH oxidizes hypoxanthine to form xanthine". Concordant with experimental data.
Reason: Core localization; the cytosol is the established compartment for XDH-mediated purine catabolism.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0005829 cytosol
TAS
Reactome:R-HSA-74258 		ACCEPT
Summary: Reactome-asserted cytosolic localization associated with the reaction "XDH oxidizes xanthine to form urate". Concordant with experimental data.
Reason: Core localization for the xanthine -> urate step of purine catabolism.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0005829 cytosol
TAS
Reactome:R-HSA-8851044 		ACCEPT
Summary: Reactome-asserted cytosolic localization associated with "BTN1A1 binds xanthine oxidoreductase (XDH)". Concordant with the cytosolic distribution.
Reason: Core localization; consistent with experimental cytosolic localization.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0005829 cytosol
TAS
Reactome:R-HSA-9727347 		ACCEPT
Summary: Reactome-asserted cytosolic localization for the reaction "XDH dehydrogenates hypoxanthine to form xanthine". Concordant with experimental data.
Reason: Core localization for XDH-mediated purine catabolism.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0005829 cytosol
TAS
Reactome:R-HSA-9727349 		ACCEPT
Summary: Reactome-asserted cytosolic localization for the reaction "XDH dehydrogenates xanthine to form urate". Concordant with experimental data.
Reason: Core localization for the xanthine -> urate dehydrogenase step.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0005829 cytosol
TAS
Reactome:R-HSA-9748991 		ACCEPT
Summary: Reactome-asserted cytosolic localization for the reaction "XDH oxidises 6MP to 6TU" (6-mercaptopurine metabolism). Concordant with the cytosolic distribution.
Reason: Core localization; consistent with the established cytosolic site of XDH.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0006149 deoxyinosine catabolic process
IDA
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocy... 		MARK AS OVER ANNOTATED
Summary: MGI-assigned IDA linking XDH to deoxyinosine catabolic process, citing the rat hepatocyte localization study. XDH acts on the downstream free base hypoxanthine, not on deoxyinosine directly; the cited paper is a localization study and does not assay deoxyinosine turnover.
Reason: Whole-pathway BP over-annotation; XDH's molecular role is xanthine/ hypoxanthine oxidation, not deoxyinosine catabolism, and the cited reference addresses subcellular localization rather than this reaction.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006204 IMP catabolic process
IDA
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocy... 		MARK AS OVER ANNOTATED
Summary: MGI-assigned IDA linking XDH to IMP catabolic process. XDH acts downstream on hypoxanthine, not on IMP itself, and the cited paper is a localization study.
Reason: Whole-pathway BP over-annotation; XDH contributes only the terminal purine-base oxidation step rather than IMP catabolism.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0070674 hypoxanthine dehydrogenase activity
IDA
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocy... 		ACCEPT
Summary: IDA for hypoxanthine dehydrogenase activity (hypoxanthine -> xanthine, the NAD+-dependent activity). This is a genuine sub-activity of XDH, consistent with its role in purine degradation.
Reason: Correct molecular function; the NAD+-dependent oxidation of hypoxanthine to xanthine is a core component of XDH dehydrogenase activity.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0005829 cytosol
IDA
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocy... 		ACCEPT
Summary: Direct immunoelectron-microscopy demonstration that XO is localized to the cytosol and not to cell organelles, with corroborating biochemical fractionation. This is the strongest evidence for the enzyme's core location.
Reason: Core localization directly demonstrated: XDH/XO is a cytosolic enzyme where it performs purine catabolism.
Supporting Evidence:
PMID:1619276
The labeling pattern was clearly over the cytosol and not on cell organelles.
GO:0009114 hypoxanthine catabolic process
IDA
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocy... 		ACCEPT
Summary: IDA for hypoxanthine catabolic process. Hypoxanthine is a direct XDH substrate (oxidized to xanthine), so this is a genuine biological process of the enzyme.
Reason: Core biological process; XDH directly oxidizes hypoxanthine, the first of its two purine-catabolic steps.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006148 inosine catabolic process
IDA
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocy... 		MARK AS OVER ANNOTATED
Summary: MGI-assigned IDA linking XDH to inosine catabolic process. XDH acts downstream on the free base hypoxanthine, not on inosine, and the cited paper is a localization study.
Reason: Whole-pathway BP over-annotation; XDH's molecular role is xanthine/ hypoxanthine oxidation, not inosine catabolism.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0000255 allantoin metabolic process
IDA
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocy... 		MARK AS OVER ANNOTATED
Summary: MGI-assigned IDA linking XDH to allantoin metabolic process. In humans urate is the terminal product of purine catabolism (humans lack urate oxidase), so allantoin is not formed; this term reflects the pathway in lower organisms.
Reason: Allantoin metabolism is downstream of urate oxidase, which humans lack; XDH produces urate as the endpoint. The term is not applicable to the human enzyme's role and the cited reference does not assay allantoin.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0006196 AMP catabolic process
IDA
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocy... 		MARK AS OVER ANNOTATED
Summary: MGI-assigned IDA linking XDH to AMP catabolic process. XDH acts downstream on hypoxanthine, not on AMP, and the cited paper is a localization study.
Reason: Whole-pathway BP over-annotation; XDH participates only in the terminal purine-base oxidation, downstream of AMP degradation.
Supporting Evidence:
PMID:1619276
Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation
GO:0005576 extracellular region
HDA
PMID:16502470
Human colostrum: identification of minor proteins in the aqu... 		KEEP AS NON CORE
Summary: High-throughput proteomic detection of XDH in the aqueous phase of human colostrum, supporting a secreted/extracellular pool of the enzyme.
Reason: Extracellular/secreted localization is genuine (milk/colostrum) but non-core relative to the cytosolic catalytic function.
Supporting Evidence:
PMID:16502470
We have investigated the low abundance proteins in the aqueous phase of human colostrum
GO:0004854 xanthine dehydrogenase activity
IDA
PMID:8670112
Cloning and expression in vitro of human xanthine dehydrogen... 		ACCEPT
Summary: Direct measurement of xanthine dehydrogenase activity in lysates of cells expressing cloned human XDH, with the enzyme predominantly in the NAD+-dependent dehydrogenase form. This is direct human evidence for the core molecular function.
Reason: Core molecular function directly demonstrated for the human enzyme.
Supporting Evidence:
PMID:8670112
The COS cells expressed the enzyme predominantly (89.8 +/- 0.3%) in the dehydrogenase form.
GO:0004855 xanthine oxidase activity
IDA
PMID:8670112
Cloning and expression in vitro of human xanthine dehydrogen... 		ACCEPT
Summary: Direct measurement of XDH/XO enzyme activity in cells expressing cloned human XDH, capturing the oxidase form of the enzyme.
Reason: Core molecular function; XDH can act as an O2-dependent oxidase, directly demonstrated for the human enzyme.
Supporting Evidence:
PMID:8670112
Significant XDH/XO enzyme activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of transfected COS cells
GO:0009115 xanthine catabolic process
IDA
PMID:8670112
Cloning and expression in vitro of human xanthine dehydrogen... 		ACCEPT
Summary: Direct evidence that human XDH performs the xanthine oxidation activity central to xanthine catabolism, from cloning and enzymatic assay of the human enzyme.
Reason: Core biological process directly supported by human enzymatic data.
Supporting Evidence:
PMID:8670112
To study the expression of human xanthine dehydrogenase/oxidase (hXDH/XO), we
GO:0043546 molybdopterin cofactor binding
IDA
PMID:17301077
Human xanthine oxidase changes its substrate specificity to ... 		ACCEPT
Summary: Crystal structure of human XOR determined in complex with molybdopterin, directly demonstrating molybdopterin cofactor binding at the catalytic center.
Reason: Core cofactor-binding function; the Mo-molybdopterin center is essential for the enzyme's catalytic activity and is directly observed in the human structure.
Supporting Evidence:
PMID:17301077
The overall molybdopterin domain structure of this mutant closely resembles that of bovine milk XOR
GO:0050660 flavin adenine dinucleotide binding
IDA
PMID:17301077
Human xanthine oxidase changes its substrate specificity to ... 		ACCEPT
Summary: Crystal structure of human XOR determined in complex with FAD, directly demonstrating FAD cofactor binding.
Reason: Core cofactor-binding function; FAD is required for electron transfer to NAD+/O2 and is directly observed in the human structure.
Supporting Evidence:
PMID:17301077
Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition
GO:0051537 2 iron, 2 sulfur cluster binding
IDA
PMID:17301077
Human xanthine oxidase changes its substrate specificity to ... 		ACCEPT
Summary: Crystal structure of human XOR determined in complex with its 2Fe-2S iron-sulfur centers, directly demonstrating [2Fe-2S] cluster binding.
Reason: Core cofactor-binding function; the two [2Fe-2S] clusters relay electrons from the molybdenum center to FAD and are directly observed in the human structure.
Supporting Evidence:
PMID:17301077
Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition
GO:0004855 xanthine oxidase activity
IDA
PMID:17301077
Human xanthine oxidase changes its substrate specificity to ... 		ACCEPT
Summary: Functional characterization of recombinant human XOR and active-site mutants, establishing the xanthine oxidase (O2-dependent) activity and its dependence on active-site residues Glu803/Arg881.
Reason: Core molecular function directly characterized for the human enzyme, including structural and mutational analysis of the active site.
Supporting Evidence:
PMID:17301077
mutation of two amino acid residues in the active site of human XOR for purine substrates results in conversion of the substrate preference to AO type
GO:0009115 xanthine catabolic process
IDA
PMID:17301077
Human xanthine oxidase changes its substrate specificity to ... 		ACCEPT
Summary: Structural and functional characterization of human XOR acting on purine substrates (xanthine/hypoxanthine), supporting its role in xanthine catabolism.
Reason: Core biological process; the enzyme's xanthine oxidation activity is the defining step of xanthine catabolism.
Supporting Evidence:
PMID:17301077
mutation of two amino acid residues in the active site of human XOR for purine substrates results in conversion of the substrate preference to AO type
GO:0042803 protein homodimerization activity
IPI
PMID:17301077
Human xanthine oxidase changes its substrate specificity to ... 		ACCEPT
Summary: The human XOR crystal structure shows the enzyme is a homodimer (chains A/B/C/D of the asymmetric unit), and UniProt annotates SUBUNIT as homodimer citing this study. This is an informative, structurally supported molecular function.
Reason: XDH is a genuine functional homodimer; unlike bare protein binding, this term is informative and directly supported by the crystal structure.
Supporting Evidence:
file:human/XDH/XDH-uniprot.txt
SUBUNIT: Homodimer. Interacts with BTN1A1 (By similarity).

Core Functions

Xanthine dehydrogenase (NAD+-dependent) activity catalysing the terminal two steps of purine catabolism, oxidizing hypoxanthine to xanthine and xanthine to urate with reduction of NAD+ to NADH.

Molecular Function:
xanthine dehydrogenase activity
Directly Involved In:
xanthine catabolic process
Cellular Locations:
cytosol
Supporting Evidence:
  • PMID:8670112
    The COS cells expressed the enzyme predominantly (89.8 +/- 0.3%) in the dehydrogenase form.
  • PMID:17301077
    mutation of two amino acid residues in the active site of human XOR for purine substrates results in conversion of the substrate preference to AO type

Xanthine oxidase (O2-dependent) activity — the proteolytically or oxidatively converted form of the enzyme — oxidizing xanthine to urate with reduction of molecular oxygen to hydrogen peroxide/superoxide, contributing to reactive oxygen species generation.

Molecular Function:
xanthine oxidase activity
Directly Involved In:
xanthine catabolic process
Cellular Locations:
cytosol
Supporting Evidence:
  • PMID:8670112
    Significant XDH/XO enzyme activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of transfected COS cells
  • PMID:17301077
    Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition

Molybdopterin (Mo-co) cofactor binding at the catalytic center, required for abstraction of electrons from the purine substrate during hydroxylation.

Molecular Function:
molybdopterin cofactor binding
Cellular Locations:
cytosol
Supporting Evidence:
  • PMID:17301077
    The overall molybdopterin domain structure of this mutant closely resembles that of bovine milk XOR

FAD cofactor binding, mediating electron transfer from the iron-sulfur relay to the terminal electron acceptor (NAD+ in the dehydrogenase form, O2 in the oxidase form).

Molecular Function:
flavin adenine dinucleotide binding
Cellular Locations:
cytosol
Supporting Evidence:
  • PMID:17301077
    Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition

Two [2Fe-2S] iron-sulfur clusters per subunit that relay electrons from the molybdenum center to FAD during catalysis.

Molecular Function:
2 iron, 2 sulfur cluster binding
Cellular Locations:
cytosol
Supporting Evidence:
  • PMID:17301077
    Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar in protein structure and prosthetic group composition

References

file:human/XDH/XDH-uniprot.txt
XDH UniProtKB entry (P47989)
GO_REF:0000002
Gene Ontology annotation through association of InterPro records with GO terms
GO_REF:0000033
Annotation inferences using phylogenetic trees
GO_REF:0000044
Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
GO_REF:0000107
Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara
GO_REF:0000120
Combined Automated Annotation using Multiple IEA Methods
PMID:1619276
Subcellular localization of xanthine oxidase in rat hepatocytes: high-resolution immunoelectron microscopic study combined with biochemical analysis.
PMID:16502470
Human colostrum: identification of minor proteins in the aqueous phase by proteomics.
PMID:17301077
Human xanthine oxidase changes its substrate specificity to aldehyde oxidase type upon mutation of amino acid residues in the active site: roles of active site residues in binding and activation of purine substrate.
PMID:32296183
A reference map of the human binary protein interactome.
PMID:8670112
Cloning and expression in vitro of human xanthine dehydrogenase/oxidase.
Reactome:R-HSA-74247
XDH oxidizes hypoxanthine to form xanthine
Reactome:R-HSA-74258
XDH oxidizes xanthine to form urate
Reactome:R-HSA-8851044
BTN1A1 binds xanthine oxidoreductase (XDH)
Reactome:R-HSA-9727347
XDH dehydrogenates hypoxanthine to form xanthine
Reactome:R-HSA-9727349
XDH dehydrogenates xanthine to form urate
Reactome:R-HSA-9748991
XDH oxidises 6MP to 6TU

Suggested Questions for Experts

Q: What determines the physiological balance between the NAD+-dependent dehydrogenase (XDH) form and the ROS-generating oxidase (XO) form in different human tissues, and how is this regulated in ischemia-reperfusion injury?

Q: What is the functional significance, if any, of the secreted/milk pool of XDH relative to its cytosolic purine-catabolic role?

Suggested Experiments

Experiment: Quantitative subcellular fractionation and immuno-EM of human tissues (liver, intestine) to resolve the contested peroxisomal and sarcoplasmic-reticulum localizations against the established cytosolic pool.

Experiment: Steady-state kinetic profiling of recombinant human XDH against hypoxanthine and xanthine in both dehydrogenase and oxidase modes to quantify the relative contribution of each step and ROS output.

📚 Additional Documentation

Notes

(XDH-notes.md)

XDH (human) — curation notes

UniProt: P47989 (XDH_HUMAN). HGNC:12805. Gene ID 7498. Chromosome 2. 1333 aa.

Verified biology (from UniProt P47989 + cached publications)

XDH is xanthine dehydrogenase/oxidase (xanthine oxidoreductase, XOR), a large
(~146 kDa monomer) cytosolic homodimer. Each subunit binds a molybdopterin
(Mo-co) center, one FAD, and two [2Fe-2S] clusters. It catalyses the last two
steps of purine catabolism:
- hypoxanthine + NAD+ + H2O -> xanthine + NADH + H+ (EC 1.17.1.4; RHEA:24670)
- xanthine + NAD+ + H2O -> urate + NADH + H+ (EC 1.17.1.4; RHEA:16669)
The enzyme exists in an NAD+-dependent dehydrogenase (XDH) form that can be
converted to an O2-dependent oxidase (XO) form — reversibly by oxidation of
sulfhydryl groups (Cys535/Cys993, Cys509/Cys1318 disulfides in oxidase form) or
irreversibly by proteolysis. The XO form produces urate + H2O2 (EC 1.17.3.2;
RHEA:21132), a source of superoxide/H2O2 (reactive oxygen species).
[file:P47989 FUNCTION "Key enzyme in purine degradation. Catalyzes the oxidation of
hypoxanthine to xanthine. Catalyzes the oxidation of xanthine to uric acid.
Contributes to the generation of reactive oxygen species."]

Localization: cytoplasm/cytosol (primary); also reported peroxisome and secreted
(detected in milk/colostrum). [file:P47989 SUBCELLULAR LOCATION]

Disease: Xanthinuria type I (XAN1, MIM:278300) — isolated XDH deficiency;
excretion of large amounts of xanthine, xanthine stones, low serum/urine uric acid.
Multiple XAN1 truncating/missense variants (e.g. R149C, C150F, P1150R, and several
C-terminal truncations). [file:P47989 DISEASE]

Pharmacology: target of gout drugs allopurinol / oxypurinol, febuxostat,
topiroxostat, tigulixostat; also metabolizes 6-mercaptopurine and activates/handles
several prodrugs (azathioprine ADME). ChEMBL CHEMBL1929; Pharos Tclin.
(Note: combined XDH + sulfite oxidase deficiency = molybdenum cofactor deficiency, a
separate MOCS-gene disorder, NOT XDH.)

Key experimental references (cached)

  • PMID:8670112 (Saksela & Raivio 1996, Biochem J) — cloned complete human
    XDH/XO cDNA; expressed in COS-1 cells; measured significant XDH/XO enzyme activity;
    enzyme expressed predominantly (89.8%) in the dehydrogenase form. Human. Supports
    GO:0004854 (XDH), GO:0004855 (XO), GO:0009115 (xanthine catabolic). CATALYTIC
    ACTIVITY ref in UniProt for EC 1.17.1.4. Abstract-only cache.
  • PMID:17301077 (Yamaguchi et al. 2007, J Biochem) — human XOR expressed in
    E. coli; crystal structure of E803V mutant at 2.6 Å in complex with FAD, 2Fe-2S,
    molybdopterin, salicylate, Ca. Active-site residues Glu803/Arg881 required for
    purine substrate; mutants gain aldehyde oxidase activity; not inhibited by
    allopurinol. UniProt cites for COFACTOR, CATALYTIC ACTIVITY, FUNCTION, SUBUNIT
    (homodimer). Supports cofactor-binding IDAs, XO activity, homodimerization.
    Abstract-only cache.
  • PMID:1619276 (Ichikawa et al. 1992, J Histochem Cytochem) — immunoEM of
    rat hepatocytes: XO labeling "clearly over the cytosol and not on cell
    organelles"; NO label over ER, Golgi, lysosomes, or peroxisomes; occasional
    mito-matrix label but no biochemical XO in mito fraction. This is the
    MGI-assigned reference for the human XDH cytosol IDA and for the array of purine
    catabolic BP IDAs (hypoxanthine/inosine/deoxyinosine/AMP/IMP catabolic, allantoin
    metabolic). Note: this study is localization only (rat); the catabolic-process
    IDAs likely derive from the enzyme's established purine-degradation role, not from
    this paper's direct assays. Abstract-only cache. NOTE: this paper's data argue
    AGAINST peroxisomal XO in rat liver — relevant to the peroxisome IEA.
  • PMID:16502470 (Palmer et al. 2006, Proteomics) — human colostrum proteomics;
    XDH identified as a minor aqueous-phase protein. Supports secreted/extracellular
    (HDA). Abstract-only.
  • PMID:32296183 (Luck et al. 2020, Nature; HuRI) — human binary interactome
    (Y2H); source of the bare "protein binding" IPI (interactor GRIP1 isoform
    Q9Y3R0-3). Full text available. Uninformative MF term.

Curation reasoning summary

  • Core MF: GO:0004854 (xanthine dehydrogenase activity) and GO:0004855
    (xanthine oxidase activity) — both with direct human IDA (8670112, 17301077).
    ACCEPT the IDA/IBA; IEA duplicates ACCEPT.
  • Core cofactor binding: molybdopterin cofactor (GO:0043546), FAD (GO:0050660),
    2Fe-2S (GO:0051537) — all with human IDA from the crystal structure (17301077).
    ACCEPT. GO:0071949 FAD binding (InterPro IEA) is a valid parent/sibling — ACCEPT.
    GO:0005506 iron ion binding and GO:0046872 metal ion binding and GO:0051536
    iron-sulfur cluster binding (InterPro IEAs) are correct but generic — ACCEPT
    (broader than the specific IDA terms; harmless).
  • Core BP: GO:0009115 (xanthine catabolic process) — human IDA. ACCEPT.
    GO:0009114 (hypoxanthine catabolic) IDA — ACCEPT as core (the hypoxanthine->xanthine
    step). The other nucleotide/nucleoside catabolic BP terms (inosine, deoxyinosine,
    AMP, IMP, adenosine, guanine, GMP, dGMP, dAMP, deoxyadenosine, deoxyguanosine
    catabolic) are pathway-context terms: XDH acts only at the terminal purine-base
    oxidation steps, not on the nucleotides/nucleosides themselves. The IDAs
    (1619276-derived) and IEAs over-attribute whole upstream catabolic pathways to XDH.
    Mark the whole-pathway nucleotide/nucleoside terms as MARK_AS_OVER_ANNOTATED or
    KEEP_AS_NON_CORE; keep hypoxanthine/xanthine as core.
  • GO:0000255 allantoin metabolic process — in humans urate is the END product
    (humans lack urate oxidase); allantoin is not made. MARK_AS_OVER_ANNOTATED
    (over-propagation from lower organisms). IDA ref (1619276) does not assay allantoin.
  • GO:0070674 hypoxanthine dehydrogenase / GO:0070675 hypoxanthine oxidase — these are
    the hypoxanthine->xanthine activities; real (human IDA 1619276 for dehydrogenase).
    ACCEPT (or KEEP_AS_NON_CORE as sub-activities of the two canonical MFs).
  • GO:0016491 oxidoreductase activity (IEA) — correct but generic parent. ACCEPT
    (harmless broad IEA) — but not core.
  • CC: GO:0005829 cytosol (IDA 1619276, TAS Reactome, IEA) — core, ACCEPT.
    GO:0005737 cytoplasm (IEA) — broad parent, ACCEPT non-core. GO:0005576
    extracellular region (IBA/IEA/HDA) — secreted/milk pool, real but minor;
    KEEP_AS_NON_CORE. GO:0005777 peroxisome (IEA by similarity) — contradicted by
    1619276 rat immunoEM; MARK_AS_OVER_ANNOTATED. GO:0016529 sarcoplasmic reticulum
    (Ensembl IEA) — weak/orthology; MARK_AS_OVER_ANNOTATED.
  • GO:0005515 protein binding IPI (HuRI) — bare, uninformative;
    MARK_AS_OVER_ANNOTATED per curation policy (do not REMOVE experimental IPI).
  • GO:0042803 protein homodimerization activity IPI (17301077) — XDH is a genuine
    homodimer; ACCEPT (informative and structurally supported).

📄 View Raw YAML

 
id: P47989
gene_symbol: XDH
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  XDH encodes xanthine dehydrogenase/oxidase (xanthine oxidoreductase, XOR), the
  molybdo-flavoenzyme that catalyses the last two steps of purine catabolism:
  the oxidation of hypoxanthine to xanthine and of xanthine to urate (uric
  acid). It is a large (~146 kDa subunit) cytosolic homodimer in which each
  subunit carries a molybdopterin (Mo-co) active-site center, one FAD, and two
  [2Fe-2S] clusters that shuttle electrons from the molybdenum center to the
  flavin. The enzyme exists as an NAD+-dependent dehydrogenase (XDH) form that
  reduces NAD+ to NADH, and can be converted to an O2-dependent oxidase (XO)
  form — reversibly through oxidation of sulfhydryl groups or irreversibly by
  proteolysis. The XO form reduces molecular oxygen to superoxide and hydrogen
  peroxide, making XDH a physiological source of reactive oxygen species. XDH is
  predominantly cytosolic and is highly expressed in liver and intestine; it is
  also secreted and detectable in milk/colostrum. Loss-of-function mutations
  cause xanthinuria type I, characterized by xanthine stones and hypouricemia,
  and XDH is the pharmacological target of the anti-gout drugs allopurinol,
  febuxostat and topiroxostat.
references:
- id: file:human/XDH/XDH-uniprot.txt
  title: XDH UniProtKB entry (P47989)
  findings: []
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:1619276
  title: 'Subcellular localization of xanthine oxidase in rat hepatocytes: high-resolution
    immunoelectron microscopic study combined with biochemical analysis.'
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Rat hepatocyte immunoelectron microscopy localizing XO to the cytosol; the
      MGI-assigned reference for the human XDH cytosol IDA and the array of purine
      catabolic-process IDAs. Directly supports cytosolic localization and argues
      against peroxisomal/organellar XO. Abstract-only in cache.
- id: PMID:16502470
  title: 'Human colostrum: identification of minor proteins in the aqueous phase by
    proteomics.'
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      Proteomics identification of XDH as a minor protein in human colostrum;
      supports the secreted/extracellular (milk) pool. Contextual, not central to
      the enzyme's core catalytic function.
- id: PMID:17301077
  title: 'Human xanthine oxidase changes its substrate specificity to aldehyde oxidase
    type upon mutation of amino acid residues in the active site: roles of active
    site residues in binding and activation of purine substrate.'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Crystal structure of human XOR (E803V mutant) in complex with FAD, 2Fe-2S,
      molybdopterin and salicylate; directly establishes the cofactor complement,
      homodimeric subunit structure, active-site purine-binding residues and
      allopurinol interaction. Cited by UniProt for COFACTOR, CATALYTIC ACTIVITY,
      FUNCTION and SUBUNIT.
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      High-throughput Y2H interactome (HuRI); source of the bare protein-binding
      IPI (interactor GRIP1 isoform Q9Y3R0-3). Systematic screen, not focused on
      XDH function; yields an uninformative molecular-function term.
- id: PMID:8670112
  title: Cloning and expression in vitro of human xanthine dehydrogenase/oxidase.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Cloning and heterologous expression of the complete human XDH/XO coding
      sequence with measured XDH/XO enzyme activity; the enzyme was expressed
      predominantly in the dehydrogenase form. UniProt CATALYTIC ACTIVITY
      reference for EC 1.17.1.4. Directly supports xanthine dehydrogenase/oxidase
      activity in human.
- id: Reactome:R-HSA-74247
  title: XDH oxidizes hypoxanthine to form xanthine
  findings: []
- id: Reactome:R-HSA-74258
  title: XDH oxidizes xanthine to form urate
  findings: []
- id: Reactome:R-HSA-8851044
  title: BTN1A1 binds xanthine oxidoreductase (XDH)
  findings: []
- id: Reactome:R-HSA-9727347
  title: XDH dehydrogenates hypoxanthine to form xanthine
  findings: []
- id: Reactome:R-HSA-9727349
  title: XDH dehydrogenates xanthine to form urate
  findings: []
- id: Reactome:R-HSA-9748991
  title: XDH oxidises 6MP to 6TU
  findings: []
existing_annotations:
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      Phylogenetically-inferred extracellular localization. XDH is a
      predominantly cytosolic enzyme but is genuinely secreted and detectable in
      milk/colostrum, so an extracellular pool exists; however, "is_active_in
      extracellular region" overstates this as a site of catalytic activity.
    action: KEEP_AS_NON_CORE
    reason: >-
      XDH is secreted and found in milk (colostrum proteomics), so an
      extracellular pool is real, but this is a minor, non-core location relative
      to the enzyme's cytosolic purine-catabolic function.
    supported_by:
    - reference_id: PMID:16502470
      supporting_text: >-
        Human colostrum: identification of minor proteins in the aqueous phase by
        proteomics.
- term:
    id: GO:0009115
    label: xanthine catabolic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      Phylogenetic inference that XDH is involved in xanthine catabolism. This is
      the canonical biological role of the enzyme (xanthine -> urate), independently
      supported by human experimental data.
    action: ACCEPT
    reason: >-
      Core biological process; the IBA agrees with human IDA evidence that XDH
      catalyses the xanthine oxidation step of purine degradation.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: >-
        To study the expression of human xanthine dehydrogenase/oxidase (hXDH/XO),
        we
- term:
    id: GO:0004854
    label: xanthine dehydrogenase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      Phylogenetically-inferred xanthine dehydrogenase (NAD+-dependent) activity,
      the defining molecular function of the XDH form of the enzyme.
    action: ACCEPT
    reason: >-
      Core molecular function, concordant with direct human enzymatic assays and
      the EC 1.17.1.4 catalytic activity annotated by UniProt.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: >-
        The COS
        cells expressed the enzyme predominantly (89.8 +/- 0.3%) in the dehydrogenase
        form.
- term:
    id: GO:0004854
    label: xanthine dehydrogenase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated (RHEA/EC 1.17.1.4-based) assignment of xanthine dehydrogenase
      activity. Duplicates the experimentally supported core molecular function.
    action: ACCEPT
    reason: >-
      Correct core molecular function; the EC 1.17.1.4 / RHEA:16669 mapping matches
      the human catalytic activity.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: >-
        Significant XDH/XO enzyme
        activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of
        transfected COS cells
- term:
    id: GO:0005506
    label: iron ion binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro-based iron ion binding. XDH binds iron as part of its two [2Fe-2S]
      clusters, so this is correct but generic relative to the specific
      cluster-binding term.
    action: ACCEPT
    reason: >-
      Correct: iron is a constituent of the [2Fe-2S] clusters directly demonstrated
      in the crystal structure. It is a broad parent of the more specific
      2 iron, 2 sulfur cluster binding term but is not incorrect.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
        in
        protein structure and prosthetic group composition
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: >-
      UniProt Subcellular Location keyword mapping (Secreted) to extracellular
      region. Consistent with the secreted milk pool.
    action: KEEP_AS_NON_CORE
    reason: >-
      Extracellular localization is real (secreted, found in milk) but non-core
      relative to the cytosolic catalytic function.
    supported_by:
    - reference_id: PMID:16502470
      supporting_text: >-
        This is the first comprehensive proteomic
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: >-
      UniProt Subcellular Location mapping to cytoplasm. Correct but a broad parent
      of the more specific and experimentally supported cytosol term.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct localization but generic; the specific cytosol annotation (with IDA
      evidence) is the informative core location.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0005777
    label: peroxisome
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: >-
      UniProt Subcellular Location keyword mapping to peroxisome (annotated in
      UniProt by similarity). Direct experimental immunoelectron microscopy of
      hepatocytes found XO over the cytosol and specifically NOT over peroxisomes.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Peroxisomal localization is contradicted by high-resolution immunoEM, which
      showed no XO labeling over peroxisomes; the human XDH is overwhelmingly
      cytosolic. The peroxisome assignment is a weak by-similarity keyword mapping.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        A few gold particles were found
        over the mitochondrial matrix, but not over the endoplasmic reticulum, Golgi
        apparatus, lysosomes, or peroxisomes, including their crystalloid core.
- term:
    id: GO:0016491
    label: oxidoreductase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated assignment of the generic oxidoreductase activity parent term.
      Correct but uninformative given the specific xanthine dehydrogenase/oxidase
      activities.
    action: KEEP_AS_NON_CORE
    reason: >-
      XDH is an oxidoreductase, but this high-level term is subsumed by the specific
      xanthine dehydrogenase and xanthine oxidase activity annotations.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: >-
        Significant XDH/XO enzyme
        activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of
        transfected COS cells
- term:
    id: GO:0043546
    label: molybdopterin cofactor binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated assignment of molybdopterin cofactor binding, matching the
      molybdenum-molybdopterin (Mo-co) active center directly observed in the human
      XOR crystal structure.
    action: ACCEPT
    reason: >-
      Core cofactor-binding function; the Mo-molybdopterin cofactor is directly
      demonstrated in the human structure and is essential for catalysis.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        The overall molybdopterin domain
        structure of this mutant closely resembles that of bovine milk XOR
- term:
    id: GO:0046872
    label: metal ion binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro-based generic metal ion binding. Correct (XDH binds molybdenum and
      iron) but a broad parent of the specific cofactor-binding terms.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct but uninformative generic term; the specific molybdopterin and
      2Fe-2S cluster binding annotations capture the actual metal cofactors.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
        in
        protein structure and prosthetic group composition
- term:
    id: GO:0050660
    label: flavin adenine dinucleotide binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated assignment of FAD binding, matching the FAD cofactor directly
      observed in the human XOR structure.
    action: ACCEPT
    reason: >-
      Core cofactor-binding function; FAD is one of the three prosthetic groups
      required for electron transfer and is directly demonstrated in the structure.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
        in
        protein structure and prosthetic group composition
- term:
    id: GO:0051536
    label: iron-sulfur cluster binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro-based iron-sulfur cluster binding. Correct but a parent of the
      specific 2 iron, 2 sulfur cluster binding term supported by the structure.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct but generic; the specific 2Fe-2S cluster binding annotation (IDA) is
      the informative term.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
        in
        protein structure and prosthetic group composition
- term:
    id: GO:0051537
    label: 2 iron, 2 sulfur cluster binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated assignment of [2Fe-2S] cluster binding, matching the two [2Fe-2S]
      centers directly observed in the human XOR structure.
    action: ACCEPT
    reason: >-
      Core cofactor-binding function; XDH binds two [2Fe-2S] clusters that relay
      electrons from the Mo center to FAD.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
        in
        protein structure and prosthetic group composition
- term:
    id: GO:0071949
    label: FAD binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro-based FAD binding, a sibling/near-synonym of the flavin adenine
      dinucleotide binding term. Correct and supported by the structure.
    action: ACCEPT
    reason: >-
      Correct cofactor-binding function; FAD is a directly observed prosthetic
      group of XDH.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
        in
        protein structure and prosthetic group composition
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  qualifier: enables
  review:
    summary: >-
      Bare "protein binding" from a high-throughput binary interactome screen
      (HuRI; interactor GRIP1 isoform). The term is uninformative and does not
      describe a specific molecular function of XDH.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Per curation guidelines, bare protein binding provides no functional insight
      and derives from a systematic Y2H screen rather than a characterized XDH
      interaction; retained (not removed) but flagged as uninformative.
    supported_by:
    - reference_id: PMID:32296183
      supporting_text: >-
        we present a human 'all-by-all'
        reference interactome map of human binary protein interactions, or 'HuRI'.
- term:
    id: GO:0004855
    label: xanthine oxidase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated (RHEA:21132 / EC 1.17.3.2-based) assignment of xanthine oxidase
      activity, the O2-dependent form of the enzyme. Duplicates the experimentally
      supported core molecular function.
    action: ACCEPT
    reason: >-
      Correct core molecular function; XDH can operate as an O2-dependent oxidase
      producing urate and H2O2, directly demonstrated in human enzyme assays.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: Cloning and expression in vitro of human xanthine dehydrogenase/oxidase.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: is_active_in
  review:
    summary: >-
      Automated assignment of cytosolic localization, concordant with the
      experimentally established primary cytosolic site of XDH activity.
    action: ACCEPT
    reason: >-
      Core localization; XDH is predominantly cytosolic where it performs purine
      catabolism, directly supported by immunoEM.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0006147
    label: guanine catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl orthology transfer of guanine catabolic process. XDH does not act on
      guanine itself (guanine is deaminated to xanthine by guanine deaminase); XDH
      contributes only the downstream xanthine oxidation step.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP transferred by orthology; XDH's molecular role is limited to
      the terminal xanthine -> urate step, not to catabolism of guanine as a substrate.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006148
    label: inosine catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: >-
      Automated assignment of inosine catabolic process. XDH does not act on
      inosine (a nucleoside); it acts downstream on hypoxanthine/xanthine.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Pathway-context over-annotation; XDH participates only in the terminal
      purine-base oxidation steps of the pathway, not in inosine catabolism per se.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006149
    label: deoxyinosine catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: >-
      Automated assignment of deoxyinosine catabolic process; XDH acts only on the
      downstream free base hypoxanthine, not on deoxyinosine.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Pathway-context over-annotation; XDH's molecular activity is xanthine/
      hypoxanthine oxidation, not deoxyinosine catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006154
    label: adenosine catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl orthology transfer of adenosine catabolic process. XDH does not act
      on adenosine; the pathway reaches XDH only after deamination/dephosphorylation
      to hypoxanthine.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH contributes only the terminal xanthine
      oxidation step, not adenosine catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006157
    label: deoxyadenosine catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl orthology transfer of deoxyadenosine catabolic process; XDH acts only
      on the free base hypoxanthine downstream.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH's role is limited to the terminal
      purine-base oxidation.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006161
    label: deoxyguanosine catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl orthology transfer of deoxyguanosine catabolic process; XDH does not
      act on deoxyguanosine.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH acts only at the terminal xanthine ->
      urate step.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006196
    label: AMP catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: >-
      Automated assignment of AMP catabolic process; XDH does not act on AMP but on
      the downstream free base hypoxanthine.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH's molecular activity is xanthine/
      hypoxanthine oxidation, downstream of AMP degradation.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006204
    label: IMP catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: >-
      Automated assignment of IMP catabolic process; XDH acts only on the free base
      hypoxanthine downstream of IMP breakdown.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH participates only in the terminal
      purine-base oxidation steps.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0009114
    label: hypoxanthine catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: >-
      Automated assignment of hypoxanthine catabolic process. Hypoxanthine is a
      direct XDH substrate (oxidized to xanthine), so this is a genuine biological
      process of the enzyme.
    action: ACCEPT
    reason: >-
      Core biological process; XDH directly oxidizes hypoxanthine to xanthine, the
      first of its two purine-catabolic steps.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0009115
    label: xanthine catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: >-
      Automated assignment of xanthine catabolic process, the canonical biological
      role of XDH (xanthine -> urate). Duplicates the experimentally supported core BP.
    action: ACCEPT
    reason: >-
      Core biological process directly supported by human enzymatic evidence.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: >-
        To study the expression of human xanthine dehydrogenase/oxidase (hXDH/XO),
        we
- term:
    id: GO:0016529
    label: sarcoplasmic reticulum
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: located_in
  review:
    summary: >-
      Ensembl orthology transfer of sarcoplasmic reticulum localization. XDH is a
      soluble cytosolic/secreted enzyme; direct immunoEM found it in the cytosol and
      not on organelles.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Weak orthology-transferred localization inconsistent with the established
      cytosolic distribution of XDH; not a canonical XDH compartment.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0046038
    label: GMP catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl orthology transfer of GMP catabolic process; XDH does not act on GMP
      but on the downstream free base xanthine.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH's role is the terminal xanthine ->
      urate step, not GMP catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0046055
    label: dGMP catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl orthology transfer of dGMP catabolic process; XDH does not act on dGMP.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH acts only at the terminal purine-base
      oxidation steps.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0046059
    label: dAMP catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ensembl orthology transfer of dAMP catabolic process; XDH does not act on dAMP.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH contributes only the terminal xanthine/
      hypoxanthine oxidation step.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0070674
    label: hypoxanthine dehydrogenase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated assignment of hypoxanthine dehydrogenase activity, the NAD+-dependent
      oxidation of hypoxanthine to xanthine. This is a genuine sub-activity of XDH
      and is directly supported by human IDA elsewhere in the record.
    action: ACCEPT
    reason: >-
      Correct molecular function; XDH oxidizes hypoxanthine to xanthine in an
      NAD+-dependent manner, a component of its dehydrogenase activity.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        mutation of two amino acid residues in the active
        site of human XOR for purine substrates results in conversion of the substrate
        preference to AO type
- term:
    id: GO:0070675
    label: hypoxanthine oxidase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: enables
  review:
    summary: >-
      Ensembl orthology transfer of hypoxanthine oxidase activity (O2-dependent
      oxidation of hypoxanthine). A genuine sub-activity of the XO form of the enzyme.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct sub-activity of XDH's oxidase form, subsumed by the canonical
      xanthine oxidase activity; retained as a valid but non-core molecular function.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        mutation of two amino acid residues in the active
        site of human XOR for purine substrates results in conversion of the substrate
        preference to AO type
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-74247
  qualifier: located_in
  review:
    summary: >-
      Reactome-asserted cytosolic localization associated with the reaction "XDH
      oxidizes hypoxanthine to form xanthine". Concordant with experimental data.
    action: ACCEPT
    reason: >-
      Core localization; the cytosol is the established compartment for XDH-mediated
      purine catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-74258
  qualifier: located_in
  review:
    summary: >-
      Reactome-asserted cytosolic localization associated with the reaction "XDH
      oxidizes xanthine to form urate". Concordant with experimental data.
    action: ACCEPT
    reason: >-
      Core localization for the xanthine -> urate step of purine catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8851044
  qualifier: located_in
  review:
    summary: >-
      Reactome-asserted cytosolic localization associated with "BTN1A1 binds
      xanthine oxidoreductase (XDH)". Concordant with the cytosolic distribution.
    action: ACCEPT
    reason: >-
      Core localization; consistent with experimental cytosolic localization.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9727347
  qualifier: located_in
  review:
    summary: >-
      Reactome-asserted cytosolic localization for the reaction "XDH dehydrogenates
      hypoxanthine to form xanthine". Concordant with experimental data.
    action: ACCEPT
    reason: >-
      Core localization for XDH-mediated purine catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9727349
  qualifier: located_in
  review:
    summary: >-
      Reactome-asserted cytosolic localization for the reaction "XDH dehydrogenates
      xanthine to form urate". Concordant with experimental data.
    action: ACCEPT
    reason: >-
      Core localization for the xanthine -> urate dehydrogenase step.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9748991
  qualifier: located_in
  review:
    summary: >-
      Reactome-asserted cytosolic localization for the reaction "XDH oxidises 6MP to
      6TU" (6-mercaptopurine metabolism). Concordant with the cytosolic distribution.
    action: ACCEPT
    reason: >-
      Core localization; consistent with the established cytosolic site of XDH.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0006149
    label: deoxyinosine catabolic process
  evidence_type: IDA
  original_reference_id: PMID:1619276
  qualifier: involved_in
  review:
    summary: >-
      MGI-assigned IDA linking XDH to deoxyinosine catabolic process, citing the rat
      hepatocyte localization study. XDH acts on the downstream free base
      hypoxanthine, not on deoxyinosine directly; the cited paper is a localization
      study and does not assay deoxyinosine turnover.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH's molecular role is xanthine/
      hypoxanthine oxidation, not deoxyinosine catabolism, and the cited reference
      addresses subcellular localization rather than this reaction.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006204
    label: IMP catabolic process
  evidence_type: IDA
  original_reference_id: PMID:1619276
  qualifier: involved_in
  review:
    summary: >-
      MGI-assigned IDA linking XDH to IMP catabolic process. XDH acts downstream on
      hypoxanthine, not on IMP itself, and the cited paper is a localization study.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH contributes only the terminal
      purine-base oxidation step rather than IMP catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0070674
    label: hypoxanthine dehydrogenase activity
  evidence_type: IDA
  original_reference_id: PMID:1619276
  qualifier: enables
  review:
    summary: >-
      IDA for hypoxanthine dehydrogenase activity (hypoxanthine -> xanthine, the
      NAD+-dependent activity). This is a genuine sub-activity of XDH, consistent
      with its role in purine degradation.
    action: ACCEPT
    reason: >-
      Correct molecular function; the NAD+-dependent oxidation of hypoxanthine to
      xanthine is a core component of XDH dehydrogenase activity.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IDA
  original_reference_id: PMID:1619276
  qualifier: is_active_in
  review:
    summary: >-
      Direct immunoelectron-microscopy demonstration that XO is localized to the
      cytosol and not to cell organelles, with corroborating biochemical
      fractionation. This is the strongest evidence for the enzyme's core location.
    action: ACCEPT
    reason: >-
      Core localization directly demonstrated: XDH/XO is a cytosolic enzyme where it
      performs purine catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        The labeling pattern was clearly
        over the cytosol and not on cell organelles.
- term:
    id: GO:0009114
    label: hypoxanthine catabolic process
  evidence_type: IDA
  original_reference_id: PMID:1619276
  qualifier: involved_in
  review:
    summary: >-
      IDA for hypoxanthine catabolic process. Hypoxanthine is a direct XDH substrate
      (oxidized to xanthine), so this is a genuine biological process of the enzyme.
    action: ACCEPT
    reason: >-
      Core biological process; XDH directly oxidizes hypoxanthine, the first of its
      two purine-catabolic steps.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006148
    label: inosine catabolic process
  evidence_type: IDA
  original_reference_id: PMID:1619276
  qualifier: involved_in
  review:
    summary: >-
      MGI-assigned IDA linking XDH to inosine catabolic process. XDH acts downstream
      on the free base hypoxanthine, not on inosine, and the cited paper is a
      localization study.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH's molecular role is xanthine/
      hypoxanthine oxidation, not inosine catabolism.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0000255
    label: allantoin metabolic process
  evidence_type: IDA
  original_reference_id: PMID:1619276
  qualifier: involved_in
  review:
    summary: >-
      MGI-assigned IDA linking XDH to allantoin metabolic process. In humans urate
      is the terminal product of purine catabolism (humans lack urate oxidase), so
      allantoin is not formed; this term reflects the pathway in lower organisms.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Allantoin metabolism is downstream of urate oxidase, which humans lack; XDH
      produces urate as the endpoint. The term is not applicable to the human
      enzyme's role and the cited reference does not assay allantoin.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0006196
    label: AMP catabolic process
  evidence_type: IDA
  original_reference_id: PMID:1619276
  qualifier: involved_in
  review:
    summary: >-
      MGI-assigned IDA linking XDH to AMP catabolic process. XDH acts downstream on
      hypoxanthine, not on AMP, and the cited paper is a localization study.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Whole-pathway BP over-annotation; XDH participates only in the terminal
      purine-base oxidation, downstream of AMP degradation.
    supported_by:
    - reference_id: PMID:1619276
      supporting_text: >-
        Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine
        degradation
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: HDA
  original_reference_id: PMID:16502470
  qualifier: located_in
  review:
    summary: >-
      High-throughput proteomic detection of XDH in the aqueous phase of human
      colostrum, supporting a secreted/extracellular pool of the enzyme.
    action: KEEP_AS_NON_CORE
    reason: >-
      Extracellular/secreted localization is genuine (milk/colostrum) but non-core
      relative to the cytosolic catalytic function.
    supported_by:
    - reference_id: PMID:16502470
      supporting_text: >-
        We have investigated the low abundance
        proteins in the aqueous phase of human colostrum
- term:
    id: GO:0004854
    label: xanthine dehydrogenase activity
  evidence_type: IDA
  original_reference_id: PMID:8670112
  qualifier: enables
  review:
    summary: >-
      Direct measurement of xanthine dehydrogenase activity in lysates of cells
      expressing cloned human XDH, with the enzyme predominantly in the
      NAD+-dependent dehydrogenase form. This is direct human evidence for the core
      molecular function.
    action: ACCEPT
    reason: >-
      Core molecular function directly demonstrated for the human enzyme.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: >-
        The COS
        cells expressed the enzyme predominantly (89.8 +/- 0.3%) in the dehydrogenase
        form.
- term:
    id: GO:0004855
    label: xanthine oxidase activity
  evidence_type: IDA
  original_reference_id: PMID:8670112
  qualifier: enables
  review:
    summary: >-
      Direct measurement of XDH/XO enzyme activity in cells expressing cloned human
      XDH, capturing the oxidase form of the enzyme.
    action: ACCEPT
    reason: >-
      Core molecular function; XDH can act as an O2-dependent oxidase, directly
      demonstrated for the human enzyme.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: >-
        Significant XDH/XO enzyme
        activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of
        transfected COS cells
- term:
    id: GO:0009115
    label: xanthine catabolic process
  evidence_type: IDA
  original_reference_id: PMID:8670112
  qualifier: involved_in
  review:
    summary: >-
      Direct evidence that human XDH performs the xanthine oxidation activity
      central to xanthine catabolism, from cloning and enzymatic assay of the human
      enzyme.
    action: ACCEPT
    reason: >-
      Core biological process directly supported by human enzymatic data.
    supported_by:
    - reference_id: PMID:8670112
      supporting_text: >-
        To study the expression of human xanthine dehydrogenase/oxidase (hXDH/XO),
        we
- term:
    id: GO:0043546
    label: molybdopterin cofactor binding
  evidence_type: IDA
  original_reference_id: PMID:17301077
  qualifier: enables
  review:
    summary: >-
      Crystal structure of human XOR determined in complex with molybdopterin,
      directly demonstrating molybdopterin cofactor binding at the catalytic center.
    action: ACCEPT
    reason: >-
      Core cofactor-binding function; the Mo-molybdopterin center is essential for
      the enzyme's catalytic activity and is directly observed in the human
      structure.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        The overall molybdopterin domain
        structure of this mutant closely resembles that of bovine milk XOR
- term:
    id: GO:0050660
    label: flavin adenine dinucleotide binding
  evidence_type: IDA
  original_reference_id: PMID:17301077
  qualifier: enables
  review:
    summary: >-
      Crystal structure of human XOR determined in complex with FAD, directly
      demonstrating FAD cofactor binding.
    action: ACCEPT
    reason: >-
      Core cofactor-binding function; FAD is required for electron transfer to
      NAD+/O2 and is directly observed in the human structure.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
        in
        protein structure and prosthetic group composition
- term:
    id: GO:0051537
    label: 2 iron, 2 sulfur cluster binding
  evidence_type: IDA
  original_reference_id: PMID:17301077
  qualifier: enables
  review:
    summary: >-
      Crystal structure of human XOR determined in complex with its 2Fe-2S
      iron-sulfur centers, directly demonstrating [2Fe-2S] cluster binding.
    action: ACCEPT
    reason: >-
      Core cofactor-binding function; the two [2Fe-2S] clusters relay electrons from
      the molybdenum center to FAD and are directly observed in the human structure.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
        in
        protein structure and prosthetic group composition
- term:
    id: GO:0004855
    label: xanthine oxidase activity
  evidence_type: IDA
  original_reference_id: PMID:17301077
  qualifier: enables
  review:
    summary: >-
      Functional characterization of recombinant human XOR and active-site mutants,
      establishing the xanthine oxidase (O2-dependent) activity and its dependence on
      active-site residues Glu803/Arg881.
    action: ACCEPT
    reason: >-
      Core molecular function directly characterized for the human enzyme, including
      structural and mutational analysis of the active site.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        mutation of two amino acid residues in the active
        site of human XOR for purine substrates results in conversion of the substrate
        preference to AO type
- term:
    id: GO:0009115
    label: xanthine catabolic process
  evidence_type: IDA
  original_reference_id: PMID:17301077
  qualifier: involved_in
  review:
    summary: >-
      Structural and functional characterization of human XOR acting on purine
      substrates (xanthine/hypoxanthine), supporting its role in xanthine catabolism.
    action: ACCEPT
    reason: >-
      Core biological process; the enzyme's xanthine oxidation activity is the
      defining step of xanthine catabolism.
    supported_by:
    - reference_id: PMID:17301077
      supporting_text: >-
        mutation of two amino acid residues in the active
        site of human XOR for purine substrates results in conversion of the substrate
        preference to AO type
- term:
    id: GO:0042803
    label: protein homodimerization activity
  evidence_type: IPI
  original_reference_id: PMID:17301077
  qualifier: enables
  review:
    summary: >-
      The human XOR crystal structure shows the enzyme is a homodimer (chains
      A/B/C/D of the asymmetric unit), and UniProt annotates SUBUNIT as homodimer
      citing this study. This is an informative, structurally supported molecular
      function.
    action: ACCEPT
    reason: >-
      XDH is a genuine functional homodimer; unlike bare protein binding, this term
      is informative and directly supported by the crystal structure.
    supported_by:
    - reference_id: file:human/XDH/XDH-uniprot.txt
      supporting_text: "SUBUNIT: Homodimer. Interacts with BTN1A1 (By similarity)."
core_functions:
- description: >-
    Xanthine dehydrogenase (NAD+-dependent) activity catalysing the terminal two
    steps of purine catabolism, oxidizing hypoxanthine to xanthine and xanthine to
    urate with reduction of NAD+ to NADH.
  molecular_function:
    id: GO:0004854
    label: xanthine dehydrogenase activity
  directly_involved_in:
  - id: GO:0009115
    label: xanthine catabolic process
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:8670112
    supporting_text: >-
      The COS
      cells expressed the enzyme predominantly (89.8 +/- 0.3%) in the dehydrogenase
      form.
  - reference_id: PMID:17301077
    supporting_text: >-
      mutation of two amino acid residues in the active
      site of human XOR for purine substrates results in conversion of the substrate
      preference to AO type
- description: >-
    Xanthine oxidase (O2-dependent) activity — the proteolytically or oxidatively
    converted form of the enzyme — oxidizing xanthine to urate with reduction of
    molecular oxygen to hydrogen peroxide/superoxide, contributing to reactive
    oxygen species generation.
  molecular_function:
    id: GO:0004855
    label: xanthine oxidase activity
  directly_involved_in:
  - id: GO:0009115
    label: xanthine catabolic process
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:8670112
    supporting_text: >-
      Significant XDH/XO enzyme
      activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of
      transfected COS cells
  - reference_id: PMID:17301077
    supporting_text: >-
      Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
      in
      protein structure and prosthetic group composition
- description: >-
    Molybdopterin (Mo-co) cofactor binding at the catalytic center, required for
    abstraction of electrons from the purine substrate during hydroxylation.
  molecular_function:
    id: GO:0043546
    label: molybdopterin cofactor binding
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:17301077
    supporting_text: >-
      The overall molybdopterin domain
      structure of this mutant closely resembles that of bovine milk XOR
- description: >-
    FAD cofactor binding, mediating electron transfer from the iron-sulfur relay to
    the terminal electron acceptor (NAD+ in the dehydrogenase form, O2 in the
    oxidase form).
  molecular_function:
    id: GO:0050660
    label: flavin adenine dinucleotide binding
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:17301077
    supporting_text: >-
      Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
      in
      protein structure and prosthetic group composition
- description: >-
    Two [2Fe-2S] iron-sulfur clusters per subunit that relay electrons from the
    molybdenum center to FAD during catalysis.
  molecular_function:
    id: GO:0051537
    label: 2 iron, 2 sulfur cluster binding
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:17301077
    supporting_text: >-
      Xanthine oxidase (oxidoreductase; XOR) and aldehyde oxidase (AO) are similar
      in
      protein structure and prosthetic group composition
proposed_new_terms: []
suggested_questions:
- question: >-
    What determines the physiological balance between the NAD+-dependent
    dehydrogenase (XDH) form and the ROS-generating oxidase (XO) form in different
    human tissues, and how is this regulated in ischemia-reperfusion injury?
- question: >-
    What is the functional significance, if any, of the secreted/milk pool of XDH
    relative to its cytosolic purine-catabolic role?
suggested_experiments:
- description: >-
    Quantitative subcellular fractionation and immuno-EM of human tissues (liver,
    intestine) to resolve the contested peroxisomal and sarcoplasmic-reticulum
    localizations against the established cytosolic pool.
- description: >-
    Steady-state kinetic profiling of recombinant human XDH against hypoxanthine
    and xanthine in both dehydrogenase and oxidase modes to quantify the relative
    contribution of each step and ROS output.