LPO

UniProt ID: P22079
Organism: Homo sapiens
Review Status: COMPLETE
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Gene Description

LPO encodes lactoperoxidase, a secreted, heme-dependent mammalian peroxidase expressed in salivary and mammary glands and in airway submucosal glands. In saliva, milk, and airway secretions, LPO uses hydrogen peroxide to oxidize thiocyanate to hypothiocyanous acid/hypothiocyanite, an antimicrobial oxidant that inhibits bacteria at mucosal surfaces. Human airway secretions contain LPO activity and support LPO-dependent antibacterial activity against several airway pathogens. LPO can also oxidize iodide and other donors under suitable chemical conditions, but thiocyanate is the best-supported physiological substrate and product partitioning depends on pH, peroxide supply, and competing substrates. The canonical precursor comprises a signal peptide at residues 1-26, a propeptide at residues 27-80, and a mature chain at residues 81-712. Human secretions contain both processed and partially unprocessed LPO, and the function of propeptide retention is unresolved. The mature enzyme binds heme and calcium and is glycosylated and disulfide-bonded. Autocatalytic peroxide-dependent covalent heme attachment and the detailed distal heme-pocket architecture are supported mainly by recombinant or non-human LPO studies; their transfer to human LPO is strongly supported by conservation but is not a direct endogenous-human structural observation. UniProt isoform 2 lacks residues 26-108, spanning the signal/ propeptide boundary and the N terminus of the canonical mature chain. Although a propeptide-deleted recombinant variant was secreted and active, the endogenous abundance, processing, localization, and physiological function of isoform 2 are not established.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0005576 extracellular region
IBA
GO_REF:0000033 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: LPO is a secreted enzyme that functions in extracellular airway, salivary, and mammary secretions.
Propagation Review
Root cause: NO FAILURE CORE
Sources checked:
MGI:MGI:1916925 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
PANTHER:PTN000895746 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
RGD:1592081 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
UniProtKB:A1KZ92 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
UniProtKB:P05164 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
UniProtKB:P22079 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
UniProtKB:P80025 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
UniProtKB:Q92626 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
WB:WBGene00004256 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
WB:WBGene00004257 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
GO:0019731 antibacterial humoral response
IBA
GO_REF:0000033 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Antibacterial humoral response directly reflects secreted LPO generation of antimicrobial oxidants.
Propagation Review
Root cause: NO FAILURE CORE
Sources checked:
PANTHER:PTN008518865 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed antibacterial humoral response assignment for human LPO.
UniProtKB:P80025 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed antibacterial humoral response assignment for human LPO.
GO:0036393 thiocyanate peroxidase activity
IBA
GO_REF:0000033 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Thiocyanate peroxidase activity is the most specific core activity of LPO, producing antimicrobial hypothiocyanous acid.
Propagation Review
Root cause: NO FAILURE CORE
Sources checked:
PANTHER:PTN008518865 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed thiocyanate peroxidase activity assignment for human LPO.
UniProtKB:P22079 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed thiocyanate peroxidase activity assignment for human LPO.
UniProtKB:P80025 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed thiocyanate peroxidase activity assignment for human LPO.
GO:0004601 peroxidase activity
IEA
GO_REF:0000120 		MODIFY
Summary: The broad peroxidase assignment should be replaced by the substrate-specific core activity.
Reason: Peroxidase activity is directionally correct but less informative than the established LPO reaction. Human LPO's core chemistry is hydrogen-peroxide- dependent thiocyanate oxidation, captured specifically by GO:0036393.
Propagation Review
Root cause: TERM SCOPING PROBLEM
Failure modes: GRANULARITY MISMATCH
Sources checked:
UniProtKB:Q5SW46 SUPPORTS TRANSFER
The source supports peroxidase catalysis, but the transferred parent term is less informative than thiocyanate peroxidase activity for human LPO.
ensembl:ENSMUSP00000099466 SUPPORTS TRANSFER
The source supports peroxidase catalysis, but the transferred parent term is less informative than thiocyanate peroxidase activity for human LPO.
InterPro:IPR010255 SUPPORTS TRANSFER
The family mapping supports peroxidase catalysis, but the parent term underspecifies human LPO's thiocyanate reaction.
Proposed replacements: thiocyanate peroxidase activity
GO:0005576 extracellular region
IEA
GO_REF:0000044 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: LPO is a secreted enzyme that functions in extracellular airway, salivary, and mammary secretions.
Propagation Review
Root cause: NO FAILURE CORE
Sources checked:
UniProtKB-SubCell:SL-0243 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
GO:0005737 cytoplasm
IEA
GO_REF:0000120 		MARK AS OVER ANNOTATED
Summary: The cytoplasm assignment is an over-propagated localization for a secretory-pathway protein.
Reason: LPO is synthesized with an N-terminal signal peptide and enters the secretory pathway before acting extracellularly. Synthesis of a precursor by cytosolic ribosomes does not establish cytoplasm as a location of the LPO protein or its activity.
Propagation Review
Root cause: PROPAGATION BAD
Failure modes: COMPARTMENT OR COMPLEX MISMATCH
Sources checked:
UniProtKB:Q5SW46 SUPPORTS SOURCE BUT NOT TARGET
The mouse donor does not establish cytoplasmic localization of human LPO.
ensembl:ENSMUSP00000099466 SUPPORTS SOURCE BUT NOT TARGET
The mouse donor does not establish cytoplasmic localization of human LPO.
UniProtKB-SubCell:SL-0086 SUPPORTS SOURCE BUT NOT TARGET
The generic cytoplasm mapping conflicts with LPO's signal-peptide-directed secretory localization.
GO:0006979 response to oxidative stress
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: The annotation overstates the demonstrated role of LPO.
Reason: LPO uses hydrogen peroxide to generate reactive antimicrobial products, but this does not by itself establish LPO as a general cellular oxidative-stress response component.
Propagation Review
Root cause: PROPAGATION BAD
Failure modes: ROLE CONFLATION
Sources checked:
InterPro:IPR010255 SUPPORTS SOURCE BUT NOT TARGET
Exact WITH/FROM source; it does not establish the overextended response to oxidative stress role for human LPO.
GO:0018969 thiocyanate metabolic process
IEA
GO_REF:0000108 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Thiocyanate metabolism directly includes LPO-catalyzed oxidation of thiocyanate to hypothiocyanous acid.
Propagation Review
Root cause: NO FAILURE CORE
Sources checked:
GO:0036393 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed thiocyanate metabolic process assignment for human LPO.
GO:0020037 heme binding
IEA
GO_REF:0000002 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Heme binding is essential to the mammalian peroxidase catalytic mechanism and is conserved in LPO.
Propagation Review
Root cause: NO FAILURE CORE
Sources checked:
InterPro:IPR010255 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed heme binding assignment for human LPO.
GO:0036393 thiocyanate peroxidase activity
IEA
GO_REF:0000116 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Thiocyanate peroxidase activity is the most specific core activity of LPO, producing antimicrobial hypothiocyanous acid.
Propagation Review
Root cause: NO FAILURE CORE
Sources checked:
RHEA:69416 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed thiocyanate peroxidase activity assignment for human LPO.
GO:0098869 cellular oxidant detoxification
IEA
GO_REF:0000120 		MARK AS OVER ANNOTATED
Summary: The annotation overstates the demonstrated role of LPO.
Reason: Consumption of hydrogen peroxide during peroxidase catalysis does not establish a general cellular oxidant-detoxification role; LPO primarily generates antimicrobial oxidants extracellularly.
Propagation Review
Root cause: PROPAGATION BAD
Failure modes: ROLE CONFLATION
Sources checked:
GO:0004601 SUPPORTS SOURCE BUT NOT TARGET
Exact WITH/FROM source; it does not establish the overextended cellular oxidant detoxification role for human LPO.
GO:0140825 SUPPORTS SOURCE BUT NOT TARGET
Exact WITH/FROM source; it does not establish the overextended cellular oxidant detoxification role for human LPO.
GO:0140825 lactoperoxidase activity
IEA
GO_REF:0000120 		KEEP AS NON CORE
Summary: Retained as experimentally plausible alternate-donor peroxidase chemistry, but not as the core LPO reaction.
Reason: GO:0140825 denotes the phenolic-donor lactoperoxidase reaction represented by RHEA:56136/EC:1.11.1.7. LPO can oxidize alternate electron donors under suitable assay conditions, but the best-supported physiological core is extracellular thiocyanate oxidation to HOSCN/OSCN rather than this exact phenolic-donor reaction.
Propagation Review
Root cause: NO FAILURE CORE
Sources checked:
RHEA:56136 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed lactoperoxidase activity assignment for human LPO.
EC:1.11.1.7 SUPPORTS TRANSFER
Exact WITH/FROM source consistent with the reviewed lactoperoxidase activity assignment for human LPO.
GO:0004601 peroxidase activity
IDA
PMID:6295491
The peroxidation of thiocyanate catalysed by myeloperoxidase... 		MODIFY
Summary: The demonstrated reaction is better represented by thiocyanate peroxidase activity.
Reason: PMID:6295491 directly studies LPO-catalyzed conversion of thiocyanate to hypothiocyanite, and this source row carries matching hydrogen peroxide, thiocyanate, water, and hypothiocyanous-acid extensions. GO:0036393 captures that chemistry more informatively than the parent peroxidase term.
Proposed replacements: thiocyanate peroxidase activity
Supporting Evidence:
PMID:6295491
Peroxidation of SCN- to OSCN-, catalysed by myeloperoxidase and lactoperoxidase, was studied.
GO:0005576 extracellular region
TAS
Reactome:R-HSA-8855490 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: LPO is a secreted enzyme that functions in extracellular airway, salivary, and mammary secretions.
GO:0005576 extracellular region
EXP
PMID:12626341
Lactoperoxidase and human airway host defense. 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Human airway secretions directly contained measurable LPO enzyme activity and LPO-sized immunoreactive bands, supporting extracellular localization.
Supporting Evidence:
PMID:12626341
The data showed that normal human airway secretions contained LPO enzyme activity (0.65 +/- 0.09 microg/mg secreted protein; n = 17), and Western blots of secretions demonstrated bands of the expected sizes for LPO.
GO:0005737 cytoplasm
ISS
GO_REF:0000024 		MARK AS OVER ANNOTATED
Summary: The cytoplasm assignment is an over-propagated localization for a secretory-pathway protein.
Reason: LPO is synthesized with an N-terminal signal peptide and enters the secretory pathway before acting extracellularly. Synthesis of a precursor by cytosolic ribosomes does not establish cytoplasm as a location of the LPO protein or its activity.
Propagation Review
Root cause: PROPAGATION BAD
Failure modes: COMPARTMENT OR COMPLEX MISMATCH
Sources checked:
UniProtKB:Q5SW46 SUPPORTS SOURCE BUT NOT TARGET
The mouse donor does not establish cytoplasmic localization of human LPO.
GO:0001580 detection of chemical stimulus involved in sensory perception of bitter taste
IDA
PMID:24248522
Salivary protein profiles are linked to bitter taste accepta... 		MARK AS OVER ANNOTATED
Summary: The annotation overstates the demonstrated role of LPO.
Reason: The study reports an association between salivary LPO abundance and infant bitter-solution acceptance, not direct chemical-stimulus detection by LPO; the causal sensory-process annotation overstates the evidence.
Supporting Evidence:
PMID:24248522
while higher abundance of bands containing lactoperoxidase, prolactin-inducible protein and S-type cystatins was associated to a higher bitterness acceptance.
GO:0005576 extracellular region
IDA
PMID:24248522
Salivary protein profiles are linked to bitter taste accepta... 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Lactoperoxidase-containing protein bands were identified from donated infant saliva, directly supporting LPO occurrence in this extracellular fluid.
Supporting Evidence:
PMID:24248522
Salivary proteins were separated by one-dimensional electrophoresis and bands were semi-quantified by image analysis. Partial least square (PLS) regression analyses were performed for each taste at both ages to explain intake ratios by band intensities. Bitterness acceptance in the younger infants was unique in the sense that salivary protein profiles could partly predict bitter taste acceptance. At that age, infants were on average indifferent to the 0.18-M urea solution, but great variability in acceptance was observed. The six bands considered as the best predictors for bitterness acceptance were identified by MALDI-TOF mass spectrometry. Higher abundance of bands containing secretory component, zinc-Ξ±-2-glycoprotein and carbonic anhydrase 6 was associated to a lower bitterness acceptance, while higher abundance of bands containing lactoperoxidase, prolactin-inducible protein and S-type cystatins was associated to a higher bitterness acceptance.
GO:0070062 extracellular exosome
HDA
PMID:19199708
Proteomic analysis of human parotid gland exosomes by multid... 		KEEP AS NON CORE
Summary: Retained as a secondary localization context.
Reason: Detection in parotid exosome preparations is a high-throughput secretion-context observation and does not establish exosome-dependent LPO function. The experimental or curator-attributed assertion is retained with curator deference.
GO:0036393 thiocyanate peroxidase activity
IDA
PMID:12626341
Lactoperoxidase and human airway host defense. 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Human airway secretions contained LPO activity together with thiocyanate at concentrations sufficient for LPO catalysis, directly supporting the substrate-specific activity.
Supporting Evidence:
PMID:12626341
SCN-, LPO's substrate, was present in undiluted airway secretions at concentrations sufficient for LPO catalysis (0.46 +/- 0.19 mM; n = 8), and diluted secretions contained antibacterial activity with LPO-like properties.
GO:0016323 basolateral plasma membrane
IDA
PMID:19059195
Molecular heterogeneity and alternative splicing of human la... 		KEEP AS NON CORE
Summary: Retained as a secondary localization context.
Reason: Immunolocalization directly placed most total-LPO signal at the basolateral aspect of airway gland cells, a context-specific localization distinct from LPO's defining extracellular catalytic activity.
Supporting Evidence:
PMID:19059195
The vast majority of the anti-rhLPO antibody signal (Fig. 6B) was in the basolateral aspect of the cells whereas anti-Ex4p1 showed that proLPO was homogeneously distributed throughout the cells (Fig. 6F).
GO:0042742 defense response to bacterium
IDA
PMID:12626341
Lactoperoxidase and human airway host defense. 		ACCEPT
Summary: Accepted as core LPO biology.
Reason: Human airway experiments demonstrate LPO-dependent antibacterial activity against multiple bacterial pathogens. The experimental or curator-attributed assertion is retained with curator deference.
Supporting Evidence:
PMID:12626341
Thus, a functional LPO system exists in human airways and may contribute to airway host defense against infection.
GO:0006979 response to oxidative stress
NAS
PMID:8964511
Cloning and sequence analysis of the human salivary peroxida... 		MARK AS OVER ANNOTATED
Summary: The annotation overstates the demonstrated role of LPO.
Reason: LPO uses hydrogen peroxide to generate reactive antimicrobial products, but this does not by itself establish LPO as a general cellular oxidative-stress response component.

Core Functions

Catalyzes the extracellular hydrogen-peroxide-dependent oxidation of thiocyanate to hypothiocyanous acid/hypothiocyanite in mucosal secretions. This oxidant-generating chemistry contributes directly to antibacterial humoral defense in saliva, milk, and the airway surface liquid. The core assignment does not extend to every alternative donor observed in vitro or to broad downstream oxidative-stress phenotypes.

Molecular Function:
thiocyanate peroxidase activity
Directly Involved In:
antibacterial humoral response
Cellular Locations:
extracellular region
Supporting Evidence:
  • PMID:6295491
    Peroxidation of SCN- to OSCN-, catalysed by myeloperoxidase and lactoperoxidase, was studied.
  • PMID:12626341
    Thus, a functional LPO system exists in human airways and may contribute to airway host defense against infection.

References

GO_REF:0000002
Gene Ontology annotation through association of InterPro records with GO terms
GO_REF:0000024
Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity
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:0000108
Automatic assignment of GO terms using logical inference, based on on inter-ontology links
GO_REF:0000116
Automatic Gene Ontology annotation based on Rhea mapping
GO_REF:0000120
Combined Automated Annotation using Multiple IEA Methods
PMID:12626341
Lactoperoxidase and human airway host defense.
  • Normal human airway secretions contain catalytically active LPO, with LPO expression localized to bronchial submucosal glands.
    "The data showed that normal human airway secretions contained LPO enzyme activity (0.65 +/- 0.09 microg/mg secreted protein; n = 17), and Western blots of secretions demonstrated bands of the expected sizes for LPO."
  • The human airway LPO system uses physiologic thiocyanate and has LPO-dependent antibacterial activity against several airway pathogens.
    "Finally, as expected based on the known antibacterial spectrum of the LPO system, airway secretions showed LPO-dependent activity against Pseudomonas aeruginosa."
PMID:19059195
Molecular heterogeneity and alternative splicing of human lactoperoxidase.
  • Human LPO produces multiple splice forms that retain the secretion signal while differing in propeptide-encoding exons.
    "Two mRNAs omit propeptide encoding exons while retaining the 5' exon encoding the secretion signal, consistent with the heterogeneity and suggesting a possible functional role for the propeptide."
  • Recombinant human LPO V1 and propeptide-skipping V3 are both secreted and catalytically active in the insect-cell expression system.
    "Both variants were secreted and were active although V1 was completely processed (see below)."
  • Human salivary LPO includes a larger, at least partially unprocessed proLPO form.
    "These data showed that a significant portion of immunoreactive LPO in secretions contains the exon 4-peptide and thus was at least partially unprocessed, in contrast to the reported N-termini for human LPO from milk determined by Edman degradation [14]."
PMID:19199708
Proteomic analysis of human parotid gland exosomes by multidimensional protein identification technology (MudPIT).
PMID:24248522
Salivary protein profiles are linked to bitter taste acceptance in infants.
  • LPO-containing salivary protein bands were detected in infant saliva and associated with greater bitter-solution acceptance, without establishing a direct LPO mechanism.
    "while higher abundance of bands containing lactoperoxidase, prolactin-inducible protein and S-type cystatins was associated to a higher bitterness acceptance."
PMID:6295491
The peroxidation of thiocyanate catalysed by myeloperoxidase and lactoperoxidase.
  • LPO catalyzes thiocyanate oxidation to hypothiocyanite with strong dependence on pH and substrate concentrations.
    "Peroxidation of SCN- to OSCN-, catalysed by myeloperoxidase and lactoperoxidase, was studied."
  • Thiocyanate can competitively inhibit LPO with respect to hydrogen peroxide under acidic conditions, bounding simple substrate models.
    "At low pH values, both SCN- and H+ inhibited myeloperoxidase and lactoperoxidase competitively with respect to H2O2."
PMID:8964511
Cloning and sequence analysis of the human salivary peroxidase-encoding cDNA.
  • A human salivary peroxidase/LPO cDNA encodes an approximately 67-kDa unglycosylated translation product.
    "In a rabbit reticulocyte lysate transcription/translation system, the cDNA produces a major protein of approx. 67 kDa, which corresponds to the calculated molecular weight of unglycosylated hSPO."
PMID:9083001
Autocatalytic processing of heme by lactoperoxidase produces the native protein-bound prosthetic group.
  • Hydrogen peroxide drives autocatalytic heme modification and covalent attachment in recombinant LPO.
    "Reaction of the purified heme. apoLPO complex with H2O2 results in both autocatalytic modification of the heme and covalent attachment to the protein."
  • Covalent heme attachment increases LPO peroxidative activity.
    "The peroxidative activity of LPO increases in proportion to the extent of covalently bound heme."
PMID:1988011
Lactoperoxidase-catalyzed oxidation of thiocyanate by hydrogen peroxide: 15N nuclear magnetic resonance and optical spectral studies.
  • Hypothiocyanite formation is a principal, condition-dependent product of the LPO/thiocyanate/hydrogen-peroxide system.
    "The formation of hypothiocyanite ion (OSCN-) as one of the oxidation products correlated well with the activity of the LPO/SCN-/H2O2 system and was maximum when the concentrations of the H2O2 and SCN- were nearly the same and the pH was less than 6.0."
  • Thiocyanate binding is required for oxidation and is lost at alkaline pH.
    "Since thiocyanate does not bind to LPO above this pH, the binding of thiocyanate to LPO is considered to be prerequisite for the oxidation of thiocyanate."
PMID:6951512
Different molecular forms of human salivary lactoperoxidase.
  • Human salivary LPO was observed in two molecular forms interpreted as a monomer and a reversible aggregate.
    "The two different molecular forms shown to exist probably represent a monomer and an aggregate."
  • Thiocyanate can promote disaggregation of salivary LPO preparations.
    "The presence of substrate (SCN-) could cause some disaggregation suggesting that the mechanism of the dissociation is influenced by the substrate."
PMID:24928513
Uric acid and thiocyanate as competing substrates of lactoperoxidase.
  • Urate is an alternative bovine-LPO substrate that competes with thiocyanate and suppresses hypothiocyanite production.
    "At physiologically relevant concentrations, urate competed effectively with thiocyanate, the main substrate of LPO for oxidation, and inhibited production of hypothiocyanite."
  • Human saliva supports peroxidase-dependent urate oxidation, providing physiological context without proving that every reaction is LPO-specific.
    "When hydrogen peroxide was added to saliva, oxidation of urate was dependent on its concentration and peroxidase activity."
PMID:18191143
Crystal structure of lactoperoxidase at 2.4 A resolution.
  • Caprine LPO is a single-chain heme peroxidase with two covalent heme ester linkages.
    "The structure confirms that the heme group is covalently linked to the protein through two ester linkages involving carboxylic groups of Glu258 and Asp108 and modified methyl groups of pyrrole rings A and C, respectively."
  • The LPO distal cavity contains a structured proton-relay path relevant to catalysis.
    "Two histidine residues and six buried water molecules are connected through a hydrogen-bonded chain from the distal heme cavity to the surface of protein molecule and seemingly form the basis of proton relay for catalytic action."
PMID:22187667
Structural evidence for the order of preference of inorganic substrates in mammalian heme peroxidases: crystal structure of the complex of lactoperoxidase with four inorganic substrates, SCN, I, Br and Cl.
  • LPO binds thiocyanate, iodide, bromide, and chloride at distinct positions on the distal heme side.
    "The structure determination of the complex of LPO with above four substrates showed that all of them occupied distinct positions in the substrate binding site on the distal heme side."
  • The crystallographic substrate positions imply a binding preference of bromide over chloride, thiocyanate, and iodide under the structural conditions used.
    "Therefore, according to the locations of four substrate anions, the order of preference for binding to lactoperoxidase is observed as Br(-) > Cl(-) > SCN(-) > I(-)."
PMID:36463312
Excess iodine exposure acutely increases salivary iodide and antimicrobial hypoiodous acid concentrations in humans.
  • Acute iodide loading in humans increases salivary iodide and hypoiodous acid, supporting hypohalous chemistry by the salivary LPO system.
    "The large iodine load delivered by the angiographic dye, several 100-fold in excess of the U.S. Recommended Daily Allowance for iodine (150Β Β΅g/day), significantly increased salivary iodide and HOI levels compared with baseline levels, whereas there was no significant change in salivary SCN- and OSCN- levels."
Reactome:R-HSA-8855490
Lactoperoxidase (LPO) produces OSCN-
  • Reactome models human LPO Compound I oxidation of thiocyanate to hypothiocyanite.
    "The Reactome event describes the halogenation cycle where LPO-derived Compound I catalyzes the oxidation of thiocyanate ion (SCN-) to hypothiocyanite ion (OSCN-)."

Suggested Questions for Experts

Q: Which molecular forms and processing states of LPO predominate in human saliva, milk, and airway surface liquid?

Q: Is UniProt isoform 2 translated and secreted endogenously, and does its residues 26-108 deletion alter heme loading, stability, or catalysis?

Q: How is covalent heme attachment established in human LPO, and what fraction of endogenous enzyme carries fully matured heme?

Q: How do physiological pH, peroxide delivery, and competing donors determine HOSCN/OSCN, HOI, and other oxidant production in each secretion?

Q: Which epithelial peroxide-generating system supplies LPO in each tissue, and is coupling based on proximity or on diffusible peroxide alone?

Q: Does reversible aggregation modulate LPO persistence or antimicrobial activity in native human secretions?

Suggested Experiments

Experiment: Apply isoform-resolved targeted proteomics and N-terminomics to fresh human saliva, milk, and airway surface liquid to quantify canonical, propeptide- retaining, and residues-26-108-deleted LPO forms.

Hypothesis: LPO processing and isoform abundance differ by secretory tissue, with canonical mature LPO predominant but measurable incompletely processed forms.

Experiment: Use CRISPR-edited human salivary- and airway-gland organoids to express each endogenous LPO splice form separately and measure secretion, heme loading, thiocyanate turnover, and bacterial killing.

Hypothesis: The residues-26-108 deletion changes LPO maturation or stability even if the secreted protein retains measurable peroxidase activity.

Experiment: Analyze affinity-purified endogenous human LPO by intact-mass spectrometry, heme-peptide mapping, and structural spectroscopy.

Hypothesis: Endogenous human LPO carries the conserved covalent heme architecture inferred from recombinant and non-human lactoperoxidases.

Experiment: Reconstitute human LPO at secretion-matched concentrations while varying pH, hydrogen-peroxide flux, thiocyanate, iodide, bromide, and urate, and quantify products by orthogonal chemical assays.

Hypothesis: Thiocyanate dominates at basal physiological concentrations, whereas donor competition and pH shift oxidant partitioning in a tissue-specific manner.

Experiment: Perturb DUOX enzymes and LPO independently in differentiated human airway and salivary organoids, then measure extracellular peroxide, HOSCN/OSCN, and pathogen survival with genetic rescue controls.

Hypothesis: Epithelial DUOX activity supplies peroxide required for LPO-dependent antimicrobial chemistry at mucosal surfaces.

Knowledge Gaps

What is not known β€” curated, literature-grounded statements of the open unknowns (the inverse of core functions).

Gap: The occupancy, covalent attachment, and maturation pathway of heme in endogenous human LPO have not been established directly; key mechanistic and structural assignments derive from recombinant or non-human LPO.

Gap: The abundance and processing of canonical LPO, incompletely processed LPO, and UniProt isoform 2 across saliva, milk, and airway secretions are unknown. Isoform 2 deletes residues 26-108, extending beyond the annotated propeptide into the canonical mature chain, so activity of a recombinant propeptide-deleted construct does not establish its endogenous function.

Gap: Physiological partitioning among thiocyanate, iodide, bromide, urate, and other donors remains incompletely quantified under the pH, peroxide flux, and substrate concentrations found in different human secretions.

Gap: The tissue-specific peroxide sources and physical or functional coupling of LPO to DUOX-family oxidases in salivary, mammary, and airway epithelia remain incompletely resolved.

Gap: The biological effects of propeptide removal or retention on LPO folding, secretion, stability, and antimicrobial activity are not known.

Gap: Human salivary preparations contain monomeric and reversibly aggregated LPO, but the native oligomeric state and physiological significance of this behavior in intact secretions are unresolved.

πŸ“š Additional Documentation

Notes

(LPO-notes.md)

LPO literature notes

Search and provenance

Provider deep research was skipped as permitted for this phase; no provider-named deep-research file was created. I audited every seeded PMID/cache and searched PubMed for primary studies covering LPO catalytic chemistry, covalent heme maturation, structure, secretion, antimicrobial activity, substrate scope, molecular forms, and human splice variants. Decisive additional records were fetched with the project's just fetch-pmid tooling. Claims below retain the experimental species and system because much of the detailed structural and kinetic literature uses non-human LPO.

Core chemistry and antimicrobial system

  • LPO is a secreted mammalian heme peroxidase that consumes hydrogen peroxide while oxidizing thiocyanate or halides. Direct product analysis showed that [PMID:1988011, "The formation of hypothiocyanite ion (OSCN-) as one of the oxidation products correlated well with the activity of the LPO/SCN-/H2O2 system and was maximum when the concentrations of the H2O2 and SCN- were nearly the same and the pH was less than 6.0."] This chemistry is condition dependent: thiocyanate binding and oxidation are lost at high pH, and excess peroxide changes the product distribution.
  • An independent kinetic study likewise examined [PMID:6295491, "Peroxidation of SCN- to OSCN-, catalysed by myeloperoxidase and lactoperoxidase"] and found sharp pH/substrate dependencies. Neither abstract identifies the LPO species, so these classic biochemical results establish enzyme chemistry but are not direct purified-human-LPO evidence.
  • The human physiological case is strongest in airway secretions. Normal airway secretions contained measured LPO activity [PMID:12626341, "The data showed that normal human airway secretions contained LPO enzyme activity (0.65 +/- 0.09 microg/mg secreted protein; n = 17), and Western blots of secretions demonstrated bands of the expected sizes for LPO."] The same study observed LPO-dependent antibacterial activity against Pseudomonas aeruginosa and activity against Burkholderia cepacia and Haemophilus influenzae. This supports mucosal host defense without implying that purified LPO alone supplies hydrogen peroxide or thiocyanate.

Heme attachment, structure, and catalytic maturation

  • Recombinant/reconstituted LPO established an autocatalytic route to the native prosthetic group: [PMID:9083001, "Reaction of the purified heme. apoLPO complex with H2O2 results in both autocatalytic modification of the heme and covalent attachment to the protein."] Activity tracked maturation because [PMID:9083001, "The peroxidative activity of LPO increases in proportion to the extent of covalently bound heme."] The abstract does not state the LPO species, so the mechanism is assigned to LPO generally rather than claimed as an endogenous human maturation experiment.
  • Caprine crystallography resolves the mature covalent architecture: [PMID:18191143, "The structure confirms that the heme group is covalently linked to the protein through two ester linkages involving carboxylic groups of Glu258 and Asp108 and modified methyl groups of pyrrole rings A and C, respectively."] The same structure identifies a distal-cavity hydrogen-bond network proposed to relay protons during catalysis. Human LPO conserves the mammalian-peroxidase architecture, but residue numbering and post-translational state should be checked against the human sequence before annotation-level use.
  • A second non-human LPO structure places SCN-, I-, Br-, and Cl- at distinct distal-side subsites [PMID:22187667, "The structure determination of the complex of LPO with above four substrates showed that all of them occupied distinct positions in the substrate binding site on the distal heme side."] Structural occupancy suggested Br- > Cl- > SCN- > I- binding under those crystallographic conditions; that order is not automatically the physiological product preference in human saliva or airway fluid.

Human secretion, splice forms, and processing

  • The cloned human salivary-peroxidase cDNA produces an approximately 67-kDa unglycosylated polypeptide in a cell-free system [PMID:8964511, "In a rabbit reticulocyte lysate transcription/translation system, the cDNA produces a major protein of approx. 67 kDa, which corresponds to the calculated molecular weight of unglycosylated hSPO."] This establishes the encoded product but not the mass of mature glycosylated secreted LPO.
  • Human LPO transcripts include variants that omit propeptide-encoding exons while retaining the signal-peptide exon [PMID:19059195, "Two mRNAs omit propeptide encoding exons while retaining the 5' exon encoding the secretion signal, consistent with the heterogeneity and suggesting a possible functional role for the propeptide."] In the baculovirus system, [PMID:19059195, "Both variants were secreted and were active although V1 was completely processed (see below)."] Therefore exon/propeptide differences do not by themselves establish distinct catalytic functions.
  • Human saliva contains a larger, incompletely processed LPO population: [PMID:19059195, "These data showed that a significant portion of immunoreactive LPO in secretions contains the exon 4-peptide and thus was at least partially unprocessed, in contrast to the reported N-termini for human LPO from milk determined by Edman degradation [14]."] The endogenous role of this processing heterogeneity remains unresolved.

Substrate and product boundaries

  • Thiocyanate is the best-supported physiological pseudohalide substrate and yields OSCN-/HOSCN, but LPO has broader chemical capacity. In bovine-LPO kinetics, urate competed with thiocyanate [PMID:24928513, "At physiologically relevant concentrations, urate competed effectively with thiocyanate, the main substrate of LPO for oxidation, and inhibited production of hypothiocyanite."] Human saliva also supported peroxidase-dependent urate oxidation, but that mixed-secretory-fluid result does not isolate LPO from other salivary peroxidases.
  • Human salivary system evidence supports iodide-to-hypoiodous-acid chemistry under a large iodine challenge: [PMID:36463312, "The large iodine load delivered by the angiographic dye, several 100-fold in excess of the U.S. Recommended Daily Allowance for iodine (150Β Β΅g/day), significantly increased salivary iodide and HOI levels compared with baseline levels, whereas there was no significant change in salivary SCN- and OSCN- levels."] This is proof of system capacity, not evidence that HOI is the dominant basal human product or purified-human-LPO specificity.
  • Chloride and bromide binding/oxidation are chemically possible for LPO, but physiological claims should account for competing SCN-, pH, peroxide supply, and species-specific kinetic evidence. Do not transfer myeloperoxidase's strong chloride/HOCl biology to LPO merely because both are mammalian peroxidases.

Molecular forms and complex boundaries

  • An early human salivary study reported two molecular forms that [PMID:6951512, "probably represent a monomer and an aggregate."] Thiocyanate could promote some disaggregation [PMID:6951512, "The presence of substrate (SCN-) could cause some disaggregation suggesting that the mechanism of the dissociation is influenced by the substrate."] The cautious wording matters: this supports preparation-dependent reversible aggregation, not an obligate physiological oligomer.
  • Broad parotid-exosome proteomics (PMID:19199708) and salivary-protein association data (PMID:24248522) support extracellular/salivary presence but do not establish that LPO is an exosome cargo with a dedicated trafficking mechanism or that LPO directly controls taste behavior.
  • Reactome R-HSA-8855490 is a useful human pathway synthesis for LPO Compound I oxidation of SCN- to OSCN-, but it is curated secondary evidence. Its statements about DUOX peroxide supply, monomeric structure, and antimicrobial effects must remain linked to the cited primary literature rather than treated as a single experiment.

Key unresolved questions

  • Which human LPO splice/processing forms dominate in milk, saliva, tears, and distinct airway gland compartments, and do they differ in stability, trafficking, or substrate preference?
  • What fraction of human secretory LPO carries fully autocatalytically matured covalent heme in vivo, and what cellular factors control that maturation?
  • Under basal human conditions, how do SCN-, iodide, bromide, urate, peroxide flux, and pH partition LPO among OSCN-/HOSCN, hypohalous, one-electron, and inactive/compound-III pathways?
  • Is salivary LPO predominantly monomeric in vivo, and when are reported aggregates physiologic rather than purification artifacts?

πŸ“„ View Raw YAML

 
id: P22079
gene_symbol: LPO
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  LPO encodes lactoperoxidase, a secreted, heme-dependent mammalian peroxidase
  expressed in salivary and mammary glands and in airway submucosal glands. In
  saliva, milk, and airway secretions, LPO uses hydrogen peroxide to oxidize
  thiocyanate to hypothiocyanous acid/hypothiocyanite, an antimicrobial oxidant
  that inhibits bacteria at mucosal surfaces. Human airway secretions contain LPO
  activity and support LPO-dependent antibacterial activity against several airway
  pathogens. LPO can also oxidize iodide and other donors under suitable chemical
  conditions, but thiocyanate is the best-supported physiological substrate and
  product partitioning depends on pH, peroxide supply, and competing substrates.

  The canonical precursor comprises a signal peptide at residues 1-26, a propeptide
  at residues 27-80, and a mature chain at residues 81-712. Human secretions contain
  both processed and partially unprocessed LPO, and the function of propeptide
  retention is unresolved. The mature enzyme binds heme and calcium and is
  glycosylated and disulfide-bonded. Autocatalytic peroxide-dependent covalent heme
  attachment and the detailed distal heme-pocket architecture are supported mainly
  by recombinant or non-human LPO studies; their transfer to human LPO is strongly
  supported by conservation but is not a direct endogenous-human structural
  observation. UniProt isoform 2 lacks residues 26-108, spanning the signal/
  propeptide boundary and the N terminus of the canonical mature chain. Although a
  propeptide-deleted recombinant variant was secreted and active, the endogenous
  abundance, processing, localization, and physiological function of isoform 2 are
  not established.
alternative_products:
- name: '1'
  id: P22079-1
- name: 2 (V3)
  id: P22079-2
  sequence_note: VSP_044473
existing_annotations:
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: LPO is a secreted enzyme that functions in extracellular airway, salivary, and mammary secretions.
    propagation_review:
      root_cause: NO_FAILURE_CORE
      source_entities:
      - source_id: MGI:MGI:1916925
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: PANTHER:PTN000895746
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: RGD:1592081
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: UniProtKB:A1KZ92
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: UniProtKB:P05164
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: UniProtKB:P22079
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: UniProtKB:P80025
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: UniProtKB:Q92626
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: WB:WBGene00004256
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
      - source_id: WB:WBGene00004257
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
  supporting_entities:
  - MGI:MGI:1916925
  - PANTHER:PTN000895746
  - RGD:1592081
  - UniProtKB:A1KZ92
  - UniProtKB:P05164
  - UniProtKB:P22079
  - UniProtKB:P80025
  - UniProtKB:Q92626
  - WB:WBGene00004256
  - WB:WBGene00004257
- term:
    id: GO:0019731
    label: antibacterial humoral response
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Antibacterial humoral response directly reflects secreted LPO generation of antimicrobial oxidants.
    propagation_review:
      root_cause: NO_FAILURE_CORE
      source_entities:
      - source_id: PANTHER:PTN008518865
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed antibacterial humoral response assignment for
          human LPO.
      - source_id: UniProtKB:P80025
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed antibacterial humoral response assignment for
          human LPO.
  supporting_entities:
  - PANTHER:PTN008518865
  - UniProtKB:P80025
- term:
    id: GO:0036393
    label: thiocyanate peroxidase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Thiocyanate peroxidase activity is the most specific core activity of LPO, producing antimicrobial hypothiocyanous
      acid.
    propagation_review:
      root_cause: NO_FAILURE_CORE
      source_entities:
      - source_id: PANTHER:PTN008518865
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed thiocyanate peroxidase activity assignment
          for human LPO.
      - source_id: UniProtKB:P22079
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed thiocyanate peroxidase activity assignment
          for human LPO.
      - source_id: UniProtKB:P80025
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed thiocyanate peroxidase activity assignment
          for human LPO.
  supporting_entities:
  - PANTHER:PTN008518865
  - UniProtKB:P22079
  - UniProtKB:P80025
- term:
    id: GO:0004601
    label: peroxidase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: The broad peroxidase assignment should be replaced by the substrate-specific core activity.
    action: MODIFY
    reason: >-
      Peroxidase activity is directionally correct but less informative than the
      established LPO reaction. Human LPO's core chemistry is hydrogen-peroxide-
      dependent thiocyanate oxidation, captured specifically by GO:0036393.
    proposed_replacement_terms:
    - id: GO:0036393
      label: thiocyanate peroxidase activity
    propagation_review:
      root_cause: TERM_SCOPING_PROBLEM
      failure_modes:
      - GRANULARITY_MISMATCH
      source_entities:
      - source_id: UniProtKB:Q5SW46
        source_status: SUPPORTS_TRANSFER
        comment: The source supports peroxidase catalysis, but the transferred parent term is less informative than thiocyanate peroxidase activity for human LPO.
      - source_id: ensembl:ENSMUSP00000099466
        source_status: SUPPORTS_TRANSFER
        comment: The source supports peroxidase catalysis, but the transferred parent term is less informative than thiocyanate peroxidase activity for human LPO.
      - source_id: InterPro:IPR010255
        source_status: SUPPORTS_TRANSFER
        comment: The family mapping supports peroxidase catalysis, but the parent term underspecifies human LPO's thiocyanate reaction.
  supporting_entities:
  - UniProtKB:Q5SW46
  - ensembl:ENSMUSP00000099466
  - InterPro:IPR010255
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: LPO is a secreted enzyme that functions in extracellular airway, salivary, and mammary secretions.
    propagation_review:
      root_cause: NO_FAILURE_CORE
      source_entities:
      - source_id: UniProtKB-SubCell:SL-0243
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed extracellular region assignment for human LPO.
  supporting_entities:
  - UniProtKB-SubCell:SL-0243
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: The cytoplasm assignment is an over-propagated localization for a secretory-pathway protein.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      LPO is synthesized with an N-terminal signal peptide and enters the secretory
      pathway before acting extracellularly. Synthesis of a precursor by cytosolic
      ribosomes does not establish cytoplasm as a location of the LPO protein or its
      activity.
    propagation_review:
      root_cause: PROPAGATION_BAD
      failure_modes:
      - COMPARTMENT_OR_COMPLEX_MISMATCH
      source_entities:
      - source_id: UniProtKB:Q5SW46
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: The mouse donor does not establish cytoplasmic localization of human LPO.
      - source_id: ensembl:ENSMUSP00000099466
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: The mouse donor does not establish cytoplasmic localization of human LPO.
      - source_id: UniProtKB-SubCell:SL-0086
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: The generic cytoplasm mapping conflicts with LPO's signal-peptide-directed secretory localization.
  supporting_entities:
  - UniProtKB:Q5SW46
  - ensembl:ENSMUSP00000099466
  - UniProtKB-SubCell:SL-0086
- term:
    id: GO:0006979
    label: response to oxidative stress
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: The annotation overstates the demonstrated role of LPO.
    action: MARK_AS_OVER_ANNOTATED
    reason: LPO uses hydrogen peroxide to generate reactive antimicrobial products, but this does not by itself
      establish LPO as a general cellular oxidative-stress response component.
    propagation_review:
      root_cause: PROPAGATION_BAD
      source_entities:
      - source_id: InterPro:IPR010255
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: Exact WITH/FROM source; it does not establish the overextended response to oxidative stress role
          for human LPO.
      failure_modes:
      - ROLE_CONFLATION
  supporting_entities:
  - InterPro:IPR010255
- term:
    id: GO:0018969
    label: thiocyanate metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  qualifier: involved_in
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Thiocyanate metabolism directly includes LPO-catalyzed oxidation of thiocyanate to hypothiocyanous acid.
    propagation_review:
      root_cause: NO_FAILURE_CORE
      source_entities:
      - source_id: GO:0036393
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed thiocyanate metabolic process assignment for
          human LPO.
  supporting_entities:
  - GO:0036393
- term:
    id: GO:0020037
    label: heme binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Heme binding is essential to the mammalian peroxidase catalytic mechanism and is conserved in LPO.
    propagation_review:
      root_cause: NO_FAILURE_CORE
      source_entities:
      - source_id: InterPro:IPR010255
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed heme binding assignment for human LPO.
  supporting_entities:
  - InterPro:IPR010255
- term:
    id: GO:0036393
    label: thiocyanate peroxidase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000116
  qualifier: enables
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Thiocyanate peroxidase activity is the most specific core activity of LPO, producing antimicrobial hypothiocyanous
      acid.
    propagation_review:
      root_cause: NO_FAILURE_CORE
      source_entities:
      - source_id: RHEA:69416
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed thiocyanate peroxidase activity assignment
          for human LPO.
  supporting_entities:
  - RHEA:69416
- term:
    id: GO:0098869
    label: cellular oxidant detoxification
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: The annotation overstates the demonstrated role of LPO.
    action: MARK_AS_OVER_ANNOTATED
    reason: Consumption of hydrogen peroxide during peroxidase catalysis does not establish a general cellular oxidant-detoxification
      role; LPO primarily generates antimicrobial oxidants extracellularly.
    propagation_review:
      root_cause: PROPAGATION_BAD
      source_entities:
      - source_id: GO:0004601
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: Exact WITH/FROM source; it does not establish the overextended cellular oxidant detoxification
          role for human LPO.
      - source_id: GO:0140825
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: Exact WITH/FROM source; it does not establish the overextended cellular oxidant detoxification
          role for human LPO.
      failure_modes:
      - ROLE_CONFLATION
  supporting_entities:
  - GO:0004601
  - GO:0140825
- term:
    id: GO:0140825
    label: lactoperoxidase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: Retained as experimentally plausible alternate-donor peroxidase chemistry, but not as the core LPO reaction.
    action: KEEP_AS_NON_CORE
    reason: >-
      GO:0140825 denotes the phenolic-donor lactoperoxidase reaction represented by
      RHEA:56136/EC:1.11.1.7. LPO can oxidize alternate electron donors under suitable
      assay conditions, but the best-supported physiological core is extracellular
      thiocyanate oxidation to HOSCN/OSCN rather than this exact phenolic-donor reaction.
    propagation_review:
      root_cause: NO_FAILURE_CORE
      source_entities:
      - source_id: RHEA:56136
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed lactoperoxidase activity assignment for human
          LPO.
      - source_id: EC:1.11.1.7
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source consistent with the reviewed lactoperoxidase activity assignment for human
          LPO.
  supporting_entities:
  - RHEA:56136
  - EC:1.11.1.7
- term:
    id: GO:0004601
    label: peroxidase activity
  evidence_type: IDA
  original_reference_id: PMID:6295491
  qualifier: enables
  review:
    summary: The demonstrated reaction is better represented by thiocyanate peroxidase activity.
    action: MODIFY
    reason: >-
      PMID:6295491 directly studies LPO-catalyzed conversion of thiocyanate to
      hypothiocyanite, and this source row carries matching hydrogen peroxide,
      thiocyanate, water, and hypothiocyanous-acid extensions. GO:0036393 captures
      that chemistry more informatively than the parent peroxidase term.
    proposed_replacement_terms:
    - id: GO:0036393
      label: thiocyanate peroxidase activity
    supported_by:
    - reference_id: PMID:6295491
      supporting_text: Peroxidation of SCN- to OSCN-, catalysed by myeloperoxidase and lactoperoxidase, was studied.
  extensions:
  - predicate: RO:0002233
    term:
      id: CHEBI:16240
      label: hydrogen peroxide
  - predicate: RO:0002233
    term:
      id: CHEBI:18022
      label: thiocyanate
  - predicate: RO:0002234
    term:
      id: CHEBI:15377
      label: water
  - predicate: RO:0002234
    term:
      id: CHEBI:133907
      label: hypothiocyanous acid
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8855490
  qualifier: located_in
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: LPO is a secreted enzyme that functions in extracellular airway, salivary, and mammary secretions.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: EXP
  original_reference_id: PMID:12626341
  qualifier: located_in
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Human airway secretions directly contained measurable LPO enzyme activity and LPO-sized immunoreactive bands, supporting extracellular localization.
    supported_by:
    - reference_id: PMID:12626341
      supporting_text: The data showed that normal human airway secretions contained LPO enzyme activity (0.65 +/- 0.09 microg/mg secreted protein; n = 17), and Western blots of secretions demonstrated bands of the expected sizes for LPO.
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: The cytoplasm assignment is an over-propagated localization for a secretory-pathway protein.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      LPO is synthesized with an N-terminal signal peptide and enters the secretory
      pathway before acting extracellularly. Synthesis of a precursor by cytosolic
      ribosomes does not establish cytoplasm as a location of the LPO protein or its
      activity.
    propagation_review:
      root_cause: PROPAGATION_BAD
      failure_modes:
      - COMPARTMENT_OR_COMPLEX_MISMATCH
      source_entities:
      - source_id: UniProtKB:Q5SW46
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: The mouse donor does not establish cytoplasmic localization of human LPO.
  supporting_entities:
  - UniProtKB:Q5SW46
- term:
    id: GO:0001580
    label: detection of chemical stimulus involved in sensory perception of bitter taste
  evidence_type: IDA
  original_reference_id: PMID:24248522
  qualifier: involved_in
  review:
    summary: The annotation overstates the demonstrated role of LPO.
    action: MARK_AS_OVER_ANNOTATED
    reason: The study reports an association between salivary LPO abundance and infant bitter-solution acceptance,
      not direct chemical-stimulus detection by LPO; the causal sensory-process annotation overstates the evidence.
    supported_by:
    - reference_id: PMID:24248522
      supporting_text: while higher abundance of bands containing lactoperoxidase, prolactin-inducible protein and S-type
        cystatins was associated to a higher bitterness acceptance.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IDA
  original_reference_id: PMID:24248522
  qualifier: located_in
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Lactoperoxidase-containing protein bands were identified from donated infant saliva, directly supporting LPO occurrence in this extracellular fluid.
    supported_by:
    - reference_id: PMID:24248522
      supporting_text: >-
        Salivary proteins were separated by one-dimensional electrophoresis and bands were semi-quantified by image analysis. Partial least square (PLS) regression analyses were performed for each taste at both ages to explain intake ratios by band intensities. Bitterness acceptance in the younger infants was unique in the sense that salivary protein profiles could partly predict bitter taste acceptance. At that age, infants were on average indifferent to the 0.18-M urea solution, but great variability in acceptance was observed. The six bands considered as the best predictors for bitterness acceptance were identified by MALDI-TOF mass spectrometry. Higher abundance of bands containing secretory component, zinc-Ξ±-2-glycoprotein and carbonic anhydrase 6 was associated to a lower bitterness acceptance, while higher abundance of bands containing lactoperoxidase, prolactin-inducible protein and S-type cystatins was associated to a higher bitterness acceptance.
  extensions:
  - predicate: BFO:0000050
    term:
      id: UBERON:0001836
      label: saliva
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:19199708
  qualifier: located_in
  review:
    summary: Retained as a secondary localization context.
    action: KEEP_AS_NON_CORE
    reason: Detection in parotid exosome preparations is a high-throughput secretion-context observation and does
      not establish exosome-dependent LPO function. The experimental or curator-attributed assertion is retained
      with curator deference.
  extensions:
  - predicate: BFO:0000050
    term:
      id: UBERON:0001831
      label: parotid gland
- term:
    id: GO:0036393
    label: thiocyanate peroxidase activity
  evidence_type: IDA
  original_reference_id: PMID:12626341
  qualifier: enables
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Human airway secretions contained LPO activity together with thiocyanate at concentrations sufficient for LPO catalysis, directly supporting the substrate-specific activity.
    supported_by:
    - reference_id: PMID:12626341
      supporting_text: SCN-, LPO's substrate, was present in undiluted airway secretions at concentrations sufficient for LPO catalysis (0.46 +/- 0.19 mM; n = 8), and diluted secretions contained antibacterial activity with LPO-like properties.
- term:
    id: GO:0016323
    label: basolateral plasma membrane
  evidence_type: IDA
  original_reference_id: PMID:19059195
  qualifier: located_in
  review:
    summary: Retained as a secondary localization context.
    action: KEEP_AS_NON_CORE
    reason: Immunolocalization directly placed most total-LPO signal at the basolateral aspect of airway gland cells, a context-specific localization distinct from LPO's defining extracellular catalytic activity.
    supported_by:
    - reference_id: PMID:19059195
      supporting_text: The vast majority of the anti-rhLPO antibody signal (Fig. 6B) was in the basolateral aspect of the cells whereas anti-Ex4p1 showed that proLPO was homogeneously distributed throughout the cells (Fig. 6F).
  extensions:
  - predicate: BFO:0000050
    term:
      id: UBERON:0003126
      label: trachea
- term:
    id: GO:0042742
    label: defense response to bacterium
  evidence_type: IDA
  original_reference_id: PMID:12626341
  qualifier: involved_in
  review:
    summary: Accepted as core LPO biology.
    action: ACCEPT
    reason: Human airway experiments demonstrate LPO-dependent antibacterial activity against multiple bacterial
      pathogens. The experimental or curator-attributed assertion is retained with curator deference.
    supported_by:
    - reference_id: PMID:12626341
      supporting_text: Thus, a functional LPO system exists in human airways and may contribute to airway host defense
        against infection.
- term:
    id: GO:0006979
    label: response to oxidative stress
  evidence_type: NAS
  original_reference_id: PMID:8964511
  qualifier: involved_in
  review:
    summary: The annotation overstates the demonstrated role of LPO.
    action: MARK_AS_OVER_ANNOTATED
    reason: LPO uses hydrogen peroxide to generate reactive antimicrobial products, but this does not by itself
      establish LPO as a general cellular oxidative-stress response component.
core_functions:
- description: >-
    Catalyzes the extracellular hydrogen-peroxide-dependent oxidation of
    thiocyanate to hypothiocyanous acid/hypothiocyanite in mucosal secretions.
    This oxidant-generating chemistry contributes directly to antibacterial
    humoral defense in saliva, milk, and the airway surface liquid. The core
    assignment does not extend to every alternative donor observed in vitro or
    to broad downstream oxidative-stress phenotypes.
  molecular_function:
    id: GO:0036393
    label: thiocyanate peroxidase activity
  directly_involved_in:
  - id: GO:0019731
    label: antibacterial humoral response
  locations:
  - id: GO:0005576
    label: extracellular region
  supported_by:
  - reference_id: PMID:6295491
    supporting_text: Peroxidation of SCN- to OSCN-, catalysed by myeloperoxidase and lactoperoxidase, was studied.
  - reference_id: PMID:12626341
    supporting_text: Thus, a functional LPO system exists in human airways and may contribute to airway host defense against infection.
knowledge_gaps:
- gap_statement: >-
    The occupancy, covalent attachment, and maturation pathway of heme in
    endogenous human LPO have not been established directly; key mechanistic and
    structural assignments derive from recombinant or non-human LPO.
- gap_statement: >-
    The abundance and processing of canonical LPO, incompletely processed LPO,
    and UniProt isoform 2 across saliva, milk, and airway secretions are unknown.
    Isoform 2 deletes residues 26-108, extending beyond the annotated propeptide
    into the canonical mature chain, so activity of a recombinant
    propeptide-deleted construct does not establish its endogenous function.
- gap_statement: >-
    Physiological partitioning among thiocyanate, iodide, bromide, urate, and
    other donors remains incompletely quantified under the pH, peroxide flux,
    and substrate concentrations found in different human secretions.
- gap_statement: >-
    The tissue-specific peroxide sources and physical or functional coupling of
    LPO to DUOX-family oxidases in salivary, mammary, and airway epithelia remain
    incompletely resolved.
- gap_statement: >-
    The biological effects of propeptide removal or retention on LPO folding,
    secretion, stability, and antimicrobial activity are not known.
- gap_statement: >-
    Human salivary preparations contain monomeric and reversibly aggregated LPO,
    but the native oligomeric state and physiological significance of this
    behavior in intact secretions are unresolved.
proposed_new_terms: []
suggested_questions:
- question: >-
    Which molecular forms and processing states of LPO predominate in human
    saliva, milk, and airway surface liquid?
- question: >-
    Is UniProt isoform 2 translated and secreted endogenously, and does its
    residues 26-108 deletion alter heme loading, stability, or catalysis?
- question: >-
    How is covalent heme attachment established in human LPO, and what fraction
    of endogenous enzyme carries fully matured heme?
- question: >-
    How do physiological pH, peroxide delivery, and competing donors determine
    HOSCN/OSCN, HOI, and other oxidant production in each secretion?
- question: >-
    Which epithelial peroxide-generating system supplies LPO in each tissue,
    and is coupling based on proximity or on diffusible peroxide alone?
- question: >-
    Does reversible aggregation modulate LPO persistence or antimicrobial
    activity in native human secretions?
suggested_experiments:
- description: >-
    Apply isoform-resolved targeted proteomics and N-terminomics to fresh human
    saliva, milk, and airway surface liquid to quantify canonical, propeptide-
    retaining, and residues-26-108-deleted LPO forms.
  hypothesis: >-
    LPO processing and isoform abundance differ by secretory tissue, with
    canonical mature LPO predominant but measurable incompletely processed forms.
- description: >-
    Use CRISPR-edited human salivary- and airway-gland organoids to express each
    endogenous LPO splice form separately and measure secretion, heme loading,
    thiocyanate turnover, and bacterial killing.
  hypothesis: >-
    The residues-26-108 deletion changes LPO maturation or stability even if the
    secreted protein retains measurable peroxidase activity.
- description: >-
    Analyze affinity-purified endogenous human LPO by intact-mass spectrometry,
    heme-peptide mapping, and structural spectroscopy.
  hypothesis: >-
    Endogenous human LPO carries the conserved covalent heme architecture inferred
    from recombinant and non-human lactoperoxidases.
- description: >-
    Reconstitute human LPO at secretion-matched concentrations while varying pH,
    hydrogen-peroxide flux, thiocyanate, iodide, bromide, and urate, and quantify
    products by orthogonal chemical assays.
  hypothesis: >-
    Thiocyanate dominates at basal physiological concentrations, whereas donor
    competition and pH shift oxidant partitioning in a tissue-specific manner.
- description: >-
    Perturb DUOX enzymes and LPO independently in differentiated human airway and
    salivary organoids, then measure extracellular peroxide, HOSCN/OSCN, and
    pathogen survival with genetic rescue controls.
  hypothesis: >-
    Epithelial DUOX activity supplies peroxide required for LPO-dependent
    antimicrobial chemistry at mucosal surfaces.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: GO reference identity and InterPro-to-GO provenance verified; it supports annotation provenance rather than direct LPO mechanism.
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of
    sequence similarity
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: GO reference identity and curator-mediated orthology-transfer provenance verified; biological applicability must be assessed at annotation level.
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: GO reference identity and PANTHER phylogenetic-inference provenance verified; it is not primary experimental evidence for human LPO.
- 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: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: GO reference identity and UniProt subcellular-location mapping provenance verified; it supports the source route rather than an independent localization experiment.
- id: GO_REF:0000108
  title: Automatic assignment of GO terms using logical inference, based on on inter-ontology links
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: GO reference identity and logical-inference provenance verified; this is an automated derivation, not direct LPO evidence.
- id: GO_REF:0000116
  title: Automatic Gene Ontology annotation based on Rhea mapping
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: GO reference identity and Rhea-reaction mapping provenance verified; reaction specificity still requires LPO biochemical evidence.
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: GO reference identity and combined automated-annotation provenance verified; it does not independently establish human LPO function.
- id: PMID:12626341
  title: Lactoperoxidase and human airway host defense.
  full_text_unavailable: true
  findings:
  - statement: Normal human airway secretions contain catalytically active LPO, with LPO expression localized to bronchial submucosal glands.
    supporting_text: The data showed that normal human airway secretions contained LPO enzyme activity (0.65 +/- 0.09 microg/mg secreted protein; n = 17), and Western blots of secretions demonstrated bands of the expected sizes for LPO.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: The human airway LPO system uses physiologic thiocyanate and has LPO-dependent antibacterial activity against several airway pathogens.
    supporting_text: Finally, as expected based on the known antibacterial spectrum of the LPO system, airway secretions showed LPO-dependent activity against Pseudomonas aeruginosa.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. This is direct human airway evidence, but the cache is abstract-only and does not resolve every component of the epithelial peroxide-generating system.
- id: PMID:19059195
  title: Molecular heterogeneity and alternative splicing of human lactoperoxidase.
  findings:
  - statement: Human LPO produces multiple splice forms that retain the secretion signal while differing in propeptide-encoding exons.
    supporting_text: Two mRNAs omit propeptide encoding exons while retaining the 5' exon encoding the secretion signal, consistent with the heterogeneity and suggesting a possible functional role for the propeptide.
    reference_section_type: ABSTRACT
  - statement: Recombinant human LPO V1 and propeptide-skipping V3 are both secreted and catalytically active in the insect-cell expression system.
    supporting_text: Both variants were secreted and were active although V1 was completely processed (see below).
    reference_section_type: RESULTS
  - statement: Human salivary LPO includes a larger, at least partially unprocessed proLPO form.
    supporting_text: These data showed that a significant portion of immunoreactive LPO in secretions contains the exon 4-peptide and thus was at least partially unprocessed, in contrast to the reported N-termini for human LPO from milk determined by Edman degradation [14].
    reference_section_type: RESULTS
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text, title, human splice products, secretion, activity, and salivary-protein observations verified. The variants were assayed heterologously, and the propeptide's endogenous function remains unresolved.
- id: PMID:19199708
  title: Proteomic analysis of human parotid gland exosomes by multidimensional protein identification technology
    (MudPIT).
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: PMID and title verified. This is a broad human parotid-exosome proteomics dataset and supports presence/provenance only; the accessible article text does not provide an LPO-specific mechanism.
- id: PMID:24248522
  title: Salivary protein profiles are linked to bitter taste acceptance in infants.
  full_text_unavailable: true
  findings:
  - statement: LPO-containing salivary protein bands were detected in infant saliva and associated with greater bitter-solution acceptance, without establishing a direct LPO mechanism.
    supporting_text: while higher abundance of bands containing lactoperoxidase, prolactin-inducible protein and S-type cystatins was associated to a higher bitterness acceptance.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. The finding is an association from mixed-protein bands, not a direct biochemical or causal function of LPO.
- id: PMID:6295491
  title: The peroxidation of thiocyanate catalysed by myeloperoxidase and lactoperoxidase.
  full_text_unavailable: true
  findings:
  - statement: LPO catalyzes thiocyanate oxidation to hypothiocyanite with strong dependence on pH and substrate concentrations.
    supporting_text: Peroxidation of SCN- to OSCN-, catalysed by myeloperoxidase and lactoperoxidase, was studied.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Thiocyanate can competitively inhibit LPO with respect to hydrogen peroxide under acidic conditions, bounding simple substrate models.
    supporting_text: At low pH values, both SCN- and H+ inhibited myeloperoxidase and lactoperoxidase competitively with respect to H2O2.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. The biochemical enzyme source is not identified in the abstract, so these kinetics should not be represented as direct human-protein evidence.
- id: PMID:8964511
  title: Cloning and sequence analysis of the human salivary peroxidase-encoding cDNA.
  full_text_unavailable: true
  findings:
  - statement: A human salivary peroxidase/LPO cDNA encodes an approximately 67-kDa unglycosylated translation product.
    supporting_text: In a rabbit reticulocyte lysate transcription/translation system, the cDNA produces a major protein of approx. 67 kDa, which corresponds to the calculated molecular weight of unglycosylated hSPO.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: PubMed title, human cDNA identity, and in-vitro translation result verified. This establishes product identity/size, not mature glycosylation, secretion, or catalytic activity.
- id: PMID:9083001
  title: Autocatalytic processing of heme by lactoperoxidase produces the native protein-bound prosthetic group.
  full_text_unavailable: true
  findings:
  - statement: Hydrogen peroxide drives autocatalytic heme modification and covalent attachment in recombinant LPO.
    supporting_text: Reaction of the purified heme. apoLPO complex with H2O2 results in both autocatalytic modification of the heme and covalent attachment to the protein.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Covalent heme attachment increases LPO peroxidative activity.
    supporting_text: The peroxidative activity of LPO increases in proportion to the extent of covalently bound heme.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. The baculovirus/reconstitution system establishes an LPO autocatalytic maturation mechanism, but the abstract does not identify the LPO species and should not be labeled direct endogenous human evidence.
- id: PMID:1988011
  title: 'Lactoperoxidase-catalyzed oxidation of thiocyanate by hydrogen peroxide: 15N nuclear magnetic resonance and optical spectral studies.'
  full_text_unavailable: true
  findings:
  - statement: Hypothiocyanite formation is a principal, condition-dependent product of the LPO/thiocyanate/hydrogen-peroxide system.
    supporting_text: The formation of hypothiocyanite ion (OSCN-) as one of the oxidation products correlated well with the activity of the LPO/SCN-/H2O2 system and was maximum when the concentrations of the H2O2 and SCN- were nearly the same and the pH was less than 6.0.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Thiocyanate binding is required for oxidation and is lost at alkaline pH.
    supporting_text: Since thiocyanate does not bind to LPO above this pH, the binding of thiocyanate to LPO is considered to be prerequisite for the oxidation of thiocyanate.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. These are direct LPO-system chemistry measurements, but enzyme species is not stated in the abstract.
- id: PMID:6951512
  title: Different molecular forms of human salivary lactoperoxidase.
  full_text_unavailable: true
  findings:
  - statement: Human salivary LPO was observed in two molecular forms interpreted as a monomer and a reversible aggregate.
    supporting_text: The two different molecular forms shown to exist probably represent a monomer and an aggregate.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Thiocyanate can promote disaggregation of salivary LPO preparations.
    supporting_text: The presence of substrate (SCN-) could cause some disaggregation suggesting that the mechanism of the dissociation is influenced by the substrate.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. This is direct human salivary evidence, but β€œmonomer and aggregate” is explicitly probabilistic and should not be converted into an obligate physiological oligomeric state.
- id: PMID:24928513
  title: Uric acid and thiocyanate as competing substrates of lactoperoxidase.
  full_text_unavailable: true
  findings:
  - statement: Urate is an alternative bovine-LPO substrate that competes with thiocyanate and suppresses hypothiocyanite production.
    supporting_text: At physiologically relevant concentrations, urate competed effectively with thiocyanate, the main substrate of LPO for oxidation, and inhibited production of hypothiocyanite.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Human saliva supports peroxidase-dependent urate oxidation, providing physiological context without proving that every reaction is LPO-specific.
    supporting_text: When hydrogen peroxide was added to saliva, oxidation of urate was dependent on its concentration and peroxidase activity.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. Direct kinetics used bovine LPO; the human-saliva experiment is mixed-system evidence and must not be presented as purified human-LPO specificity.
- id: PMID:18191143
  title: Crystal structure of lactoperoxidase at 2.4 A resolution.
  full_text_unavailable: true
  findings:
  - statement: Caprine LPO is a single-chain heme peroxidase with two covalent heme ester linkages.
    supporting_text: The structure confirms that the heme group is covalently linked to the protein through two ester linkages involving carboxylic groups of Glu258 and Asp108 and modified methyl groups of pyrrole rings A and C, respectively.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: The LPO distal cavity contains a structured proton-relay path relevant to catalysis.
    supporting_text: Two histidine residues and six buried water molecules are connected through a hydrogen-bonded chain from the distal heme cavity to the surface of protein molecule and seemingly form the basis of proton relay for catalytic action.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. This is caprine structural evidence; conserved human residues support cautious transfer, but species-specific post-translational details are not directly established.
- id: PMID:22187667
  title: 'Structural evidence for the order of preference of inorganic substrates in mammalian heme peroxidases: crystal structure of the complex of lactoperoxidase with four inorganic substrates, SCN, I, Br and Cl.'
  full_text_unavailable: true
  findings:
  - statement: LPO binds thiocyanate, iodide, bromide, and chloride at distinct positions on the distal heme side.
    supporting_text: The structure determination of the complex of LPO with above four substrates showed that all of them occupied distinct positions in the substrate binding site on the distal heme side.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: The crystallographic substrate positions imply a binding preference of bromide over chloride, thiocyanate, and iodide under the structural conditions used.
    supporting_text: Therefore, according to the locations of four substrate anions, the order of preference for binding to lactoperoxidase is observed as Br(-) > Cl(-) > SCN(-) > I(-).
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: PubMed title and abstract verified. This is non-human LPO crystallography and describes binding-site occupancy/preferences, not necessarily physiological product flux in human secretions.
- id: PMID:36463312
  title: Excess iodine exposure acutely increases salivary iodide and antimicrobial hypoiodous acid concentrations in humans.
  findings:
  - statement: Acute iodide loading in humans increases salivary iodide and hypoiodous acid, supporting hypohalous chemistry by the salivary LPO system.
    supporting_text: The large iodine load delivered by the angiographic dye, several 100-fold in excess of the U.S. Recommended Daily Allowance for iodine (150Β Β΅g/day), significantly increased salivary iodide and HOI levels compared with baseline levels, whereas there was no significant change in salivary SCN- and OSCN- levels.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Full cached article, PubMed title, and human saliva measurements verified. This is a system-level intervention with supraphysiologic iodide exposure, not purified human-LPO kinetics or evidence that HOI dominates normally.
- id: Reactome:R-HSA-8855490
  title: Lactoperoxidase (LPO) produces OSCN-
  findings:
  - statement: Reactome models human LPO Compound I oxidation of thiocyanate to hypothiocyanite.
    supporting_text: The Reactome event describes the halogenation cycle where LPO-derived Compound I catalyzes the oxidation of thiocyanate ion (SCN-) to hypothiocyanite ion (OSCN-).
    reference_section_type: OTHER
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Cached human Reactome event and participant narrative verified. This is curated pathway synthesis supported by cited literature, not an independent primary experiment.