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.
| 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). |
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Download this section (compressed HTML)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?
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.
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