Inter-alpha-trypsin inhibitor heavy chain H2 is a secreted plasma protein that is a component of the inter-alpha-inhibitor (IΞ±I) protease inhibitor complex. ITIH2 acts as a carrier and regulatory protein for hyaluronan in the extracellular matrix, participating in ECM stabilization through TSG-6-mediated transfer of heavy chains to hyaluronan. The protein contains VIT and VWFA domains, is linked to bikunin via chondroitin sulfate, and is phosphorylated by FAM20C in the extracellular space.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004867 serine-type endopeptidase inhibitor activity | IEA GO_REF:0000120 | REMOVE | Summary: This IEA annotation is based on InterPro domain mapping and UniProtKB keyword assignment. While ITIH2 is part of the inter-alpha-inhibitor complex, the actual serine protease inhibitor activity resides in the bikunin subunit, not in the ITIH2 heavy chain itself. Reason: ITIH2 (the heavy chain H2) does not possess intrinsic serine-type endopeptidase inhibitor activity. The inhibitory activity of the inter-alpha-inhibitor complex comes from the bikunin light chain, not from the heavy chains. As stated in PMID:2476436, "each protein contained a single, identical, trypsin-inhibitory chain of 30,000 Da" referring to bikunin, while "Inter-alpha-trypsin inhibitor contains noninhibitory heavy chains of 65,000 and 70,000 Da." ITIH2 is one of these noninhibitory heavy chains. The deep research confirms "ITIH2 acts as a serine-type endopeptidase inhibitor" is misleading - it's the bikunin component that has this activity. Supporting Evidence: PMID:2476436 Analysis of the proteins, the separated chains, and proteolytic derivatives thereof revealed that each protein contained a single, identical, trypsin-inhibitory chain of 30,000 Da. Inter-alpha-trypsin inhibitor contains noninhibitory heavy chains of 65,000 and 70,000 Da file:human/ITIH2/ITIH2-deep-research-falcon.md protease inhibition is primarily attributed to bikunin, while the best-characterized function of heavy chains (including ITIH2/HC2) is structural, namely covalent modification of HA in extracellular matrices |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: This IEA annotation is based on UniProtKB subcellular location vocabulary mapping. ITIH2 is indeed a secreted protein found in plasma and extracellular fluids. However, this term is very broad and less informative than more specific cellular component annotations available. Reason: While this annotation is correct, it is quite general. ITIH2 is secreted into plasma and the extracellular space as confirmed by UniProt ("Secreted") and deep research showing it is a "secreted plasma protein found in the extracellular space." The annotation is supported by experimental evidence (PMID:14718574) which identified ITIH2 in the human plasma proteome. This term is acceptable as a broad localization statement, though more specific terms like "extracellular matrix" provide better functional context. Supporting Evidence: PMID:14718574 The human plasma proteome: a nonredundant list developed by combination of four separate sources |
| GO:0030212 hyaluronan metabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: This IEA annotation is based on InterPro domain mapping. ITIH2 plays a crucial role in hyaluronan biology through TSG-6-mediated transfer of heavy chains to hyaluronan, stabilizing the ECM and regulating hyaluronan localization and interactions. Reason: This annotation accurately captures a core function of ITIH2. The protein is directly involved in hyaluronan metabolic processes through its interactions with hyaluronan. PMID:20463016 demonstrates that "TSG-6/HC2 transfer HCs from bikunin proteins to HA" and shows "a dynamic shuffling of the HCs occur in vivo" between glycosaminoglycans including hyaluronan. The deep research confirms ITIH2 "binds and stabilizes hyaluronan in the ECM" and is "covalently linking to hyaluronan, mediated by TSG-6." UniProt states ITIH2 may act as "a binding protein between hyaluronan and other matrix protein...to regulate the localization, synthesis and degradation of hyaluronan." Supporting Evidence: PMID:20463016 The heavy chain (HC) subunits of the bikunin proteins are covalently attached to a single chondroitin sulfate (CS) chain originating from bikunin and can be transferred to different hyaluronan (HA) molecules by TSG-6/HC2 file:human/ITIH2/ITIH2-uniprot.txt May act as a carrier of hyaluronan in serum or as a binding protein between hyaluronan and other matrix protein, including those on cell surfaces in tissues to regulate the localization, synthesis and degradation of hyaluronan PMID:39149600 TSG-6 is essential for the interaction with HA because it facilitates two following ester exchange reactions: it binds HC1 or HC2 of IΞ±I family covalently and then moves them to the HA fraction in this complex, where the heavy chain conjugates and releases free TSG-6 |
| GO:0030414 peptidase inhibitor activity | IEA GO_REF:0000043 | REMOVE | Summary: This IEA annotation is based on UniProtKB keyword mapping. Like GO:0004867, this annotation incorrectly attributes the protease inhibitor activity of the inter-alpha-inhibitor complex to the ITIH2 heavy chain, when it actually resides in the bikunin subunit. Reason: ITIH2 itself does not have peptidase inhibitor activity. This is the same fundamental issue as GO:0004867. The heavy chains (including ITIH2/HC2) are structural components that are linked to bikunin via chondroitin sulfate, but they do not possess enzymatic inhibitor activity. PMID:2476436 clearly identifies bikunin as "the trypsin-inhibitory chain" while describing ITIH2 and other heavy chains as "noninhibitory." The annotation conflates the complex's activity with ITIH2's individual molecular function. Supporting Evidence: PMID:2476436 Inter-alpha-trypsin inhibitor contains noninhibitory heavy chains of 65,000 and 70,000 Da file:human/ITIH2/ITIH2-deep-research-falcon.md This is not an enzyme in the classical "substrate β product" sense; rather, ITIH2 contributes a reactive heavy chain that becomes part of the HA polymer's covalent decoration |
| GO:0005515 protein binding | IPI PMID:16873769 TSG-6 potentiates the antitissue kallikrein activity of inte... | MODIFY | Summary: This IPI annotation is based on experimental evidence showing ITIH2 interaction with bikunin in the context of TSG-6-mediated complex formation. While technically correct, the generic "protein binding" term is uninformative and should be replaced with more specific molecular function terms. Reason: PMID:16873769 demonstrates that TSG-6 interacts with heavy chains from inter-alpha-inhibitor, stating "TSG-6 forms covalent complexes with heavy chains (HCs) from inter-alpha-inhibitor and pre-alpha-inhibitor and associates noncovalently with their common bikunin chain." While this confirms protein-protein interaction, the term "protein binding" is too vague per curation guidelines. ITIH2's role is better captured by more specific terms describing its function in hyaluronan binding and ECM organization rather than generic protein binding. Proposed replacements: hyaluronic acid binding Supporting Evidence: PMID:16873769 TSG-6 (the protein product of TNF-stimulated gene-6), an inflammation-associated protein, forms covalent complexes with heavy chains (HCs) from inter-alpha-inhibitor and pre-alpha-inhibitor and associates noncovalently with their common bikunin chain |
| GO:0005515 protein binding | IPI PMID:20463016 The TSG-6/HC2-mediated transfer is a dynamic process shuffli... | MODIFY | Summary: This IPI annotation is based on experimental evidence showing ITIH2 interaction with TSG-6 and involvement in the dynamic transfer of heavy chains between glycosaminoglycans. Like the previous protein binding annotation, this is too generic. Reason: PMID:20463016 demonstrates extensive protein-protein interactions in the context of TSG-6/HC2-mediated transfer, showing "TSG-6 and HC2 transfer HCs from bikunin proteins to HA in a reaction that involves two sequential transesterifications." While protein binding is occurring, this generic term does not capture the specific molecular function. The paper demonstrates hyaluronan binding activity and dynamic interactions with glycosaminoglycans, which are better represented by GO:0005540 (hyaluronic acid binding). Proposed replacements: hyaluronic acid binding Supporting Evidence: PMID:20463016 In concert, TSG-6 and HC2 transfer HCs from bikunin proteins to HA in a reaction that involves two sequential transesterifications |
| GO:0005540 hyaluronic acid binding | IDA PMID:20463016 The TSG-6/HC2-mediated transfer is a dynamic process shuffli... | ACCEPT | Summary: This IDA annotation is based on direct experimental evidence demonstrating ITIH2's ability to bind hyaluronan through TSG-6-mediated transfer reactions. This represents a core molecular function of ITIH2. Reason: This annotation accurately captures a core molecular function of ITIH2. PMID:20463016 directly demonstrates that "heavy chain (HC) subunits of the bikunin proteins...can be transferred to different hyaluronan (HA) molecules by TSG-6/HC2" and shows "HCΒ·HA complex" formation. The paper demonstrates reversible binding, showing "HCs transferred to HA may function as HC donors in subsequent transfer reactions" and "a dynamic shuffling of the HCs occur in vivo." The deep research confirms "ITIH2 binds and stabilizes hyaluronan in the ECM, often via TSG-6-mediated transfer." This is a well-supported, specific molecular function annotation. Supporting Evidence: PMID:20463016 The heavy chain (HC) subunits of the bikunin proteins are covalently attached to a single chondroitin sulfate (CS) chain originating from bikunin and can be transferred to different hyaluronan (HA) molecules by TSG-6/HC2 file:human/ITIH2/ITIH2-deep-research-perplexity-lite.md ITIH2 binds and stabilizes hyaluronan in the ECM, often via TSG-6-mediated transfer PMID:39149600 HCs have been found to function as structural proteins that can directly cross-link HA that is secreted file:human/ITIH2/ITIH2-deep-research-falcon.md ITIH2's primary molecular function is best summarized as: Extracellular structural modification of hyaluronan (HA) through the provision of HC2, which can become covalently attached to HA to form HCΒ·HA (SHAPβHA) complexes that stabilize and organize HA-rich extracellular matrices |
| GO:0031012 extracellular matrix | HDA PMID:28327460 Comprehensive proteomic characterization of stem cell-derive... | ACCEPT | Summary: This HDA annotation is based on high-throughput detection in extracellular matrix preparations from stem cell-derived matrices. ITIH2 is a bona fide ECM component through its role in stabilizing hyaluronan. Reason: ITIH2 is a well-established component of the extracellular matrix. The deep research confirms "ITIH2 participates in ECM stabilization by covalently linking to hyaluronan, mediated by TSG-6" and describes it as involved in "ECM stabilization." UniProt states ITIH2 may act as "a binding protein between hyaluronan and other matrix protein." The HDA evidence from proteomics studies of ECM is appropriate for this localization, as ITIH2 functions in the ECM compartment through its interactions with hyaluronan and other matrix components. Supporting Evidence: file:human/ITIH2/ITIH2-deep-research-perplexity-lite.md ITIH2 participates in ECM stabilization by covalently linking to hyaluronan, mediated by TSG-6 PMID:28327460 Epub 2017 Mar 7. Comprehensive proteomic characterization of stem cell-derived extracellular matrices. PMID:39149600 Through the formation of covalent connections with hyaluronic acid (HA), the inter-Ξ±-trypsin inhibitor (IΞ±I) family collaborates to preserve the stability of the extracellular matrix (ECM) |
| GO:0031012 extracellular matrix | HDA PMID:28675934 Characterization of the Extracellular Matrix of Normal and D... | ACCEPT | Summary: This HDA annotation is based on proteomic characterization of normal and diseased tissue ECM. This is consistent with ITIH2's role as an ECM-stabilizing protein. Reason: This is the same cellular component as the previous annotation but from a different proteomic study. The evidence supports ITIH2's localization to the extracellular matrix. Multiple independent HDA studies identifying ITIH2 in ECM preparations strengthens confidence in this localization annotation. Supporting Evidence: file:human/ITIH2/ITIH2-deep-research-perplexity-lite.md ITIH2 is a secreted plasma protein found in the extracellular space, especially in tissues rich in ECM PMID:28675934 Characterization of the Extracellular Matrix of Normal and Diseased Tissues Using Proteomics. |
| GO:0031012 extracellular matrix | HDA PMID:25037231 Extracellular matrix signatures of human primary metastatic ... | ACCEPT | Summary: This HDA annotation is from a study of ECM signatures in colon cancer and liver metastases. ITIH2 downregulation in cancer is consistent with its tumor suppressor role. Reason: This annotation from ECM proteomic analysis of cancer tissues is valid. The deep research notes "ITIH2 is downregulated in multiple solid tumors (breast, colon, lung), correlating with loss of tumor suppressor activity and increased invasiveness." The identification of ITIH2 in ECM preparations from cancer tissues, even if at altered levels, confirms its ECM localization and is consistent with its biological role. Supporting Evidence: file:human/ITIH2/ITIH2-deep-research-perplexity-lite.md ITIH2 is downregulated in multiple solid tumors (breast, colon, lung), correlating with loss of tumor suppressor activity and increased invasiveness PMID:25037231 Extracellular matrix signatures of human primary metastatic colon cancers and their metastases to liver. PMID:39149600 Strong evidence suggests that genes in the ITIH family may be tumor suppressors because these genes are highly down-regulated in a range of human solid tumors, including lung cancer, breast cancer and colon cancer |
| GO:0005788 endoplasmic reticulum lumen | TAS Reactome:R-HSA-8952289 | REMOVE | Summary: This TAS annotation is based on Reactome pathway annotation for FAM20C phosphorylation of substrates. While ITIH2 is phosphorylated by FAM20C, this kinase is an extracellular kinase, not an ER-localized one. Reason: This annotation appears to be incorrect. The Reactome pathway R-HSA-8952289 describes "FAM20C phosphorylates FAM20C substrates" and notes that "Extracellular serine/threonine protein kinase FAM20C is an extracellular kinase." UniProt confirms ITIH2 is "Phosphorylated by FAM20C in the extracellular medium." ITIH2 is a secreted protein that functions in the extracellular space and is phosphorylated there, not in the ER lumen. While ITIH2 transiently passes through the ER during secretion, annotation to ER lumen does not represent a functionally relevant localization for this mature, secreted protein. Supporting Evidence: Reactome:R-HSA-8952289 Extracellular serine/threonine protein kinase FAM20C is an extracellular kinase that can phosphorylate a broad range of secreted protein file:human/ITIH2/ITIH2-uniprot.txt Phosphorylated by FAM20C in the extracellular medium |
| GO:0070062 extracellular exosome | HDA PMID:23533145 In-depth proteomic analyses of exosomes isolated from expres... | KEEP AS NON CORE | Summary: This HDA annotation is based on proteomic analysis of exosomes from prostatic secretions. While ITIH2 was detected in these preparations, this likely represents contamination with abundant plasma proteins rather than specific exosomal localization. Reason: ITIH2 is an abundant plasma protein, and its detection in exosome preparations may reflect plasma contamination rather than bona fide exosomal localization. Exosomes are secreted in biological fluids that also contain plasma proteins, making it difficult to distinguish true exosomal components from co-purifying plasma proteins in HDA studies. While ITIH2 may be present in exosomal fractions, this does not represent a core functional localization. The primary and functionally relevant localization of ITIH2 is in plasma and the ECM where it stabilizes hyaluronan. This annotation should be kept but marked as non-core. Supporting Evidence: file:human/ITIH2/ITIH2-deep-research-perplexity-lite.md ITIH2 is a secreted plasma protein found in the extracellular space PMID:23533145 2013 Apr 23. In-depth proteomic analyses of exosomes isolated from expressed prostatic secretions in urine. |
| GO:0072562 blood microparticle | HDA PMID:22516433 Proteomic analysis of microvesicles from plasma of healthy d... | KEEP AS NON CORE | Summary: This HDA annotation is from proteomic analysis of blood microparticles. Similar to exosomal annotations, this likely reflects the presence of abundant plasma proteins rather than specific microparticle association. Reason: ITIH2 is a major plasma protein present at high concentrations in blood. Detection in blood microparticle preparations likely reflects the plasma milieu in which these particles exist rather than specific functional association with microparticles. The title of PMID:22516433 indicates "high individual variability" in the proteomic analysis, suggesting some non-specific associations. While this annotation may be technically correct, it does not represent a core functional localization for ITIH2, whose primary functions are in plasma and ECM. Supporting Evidence: file:human/ITIH2/ITIH2-uniprot.txt Plasma [tissue specificity] PMID:22516433 Epub 2012 Apr 10. Proteomic analysis of microvesicles from plasma of healthy donors reveals high individual variability. |
| GO:0070062 extracellular exosome | HDA PMID:20458337 MHC class II-associated proteins in B-cell exosomes and pote... | KEEP AS NON CORE | Summary: This is another HDA annotation for extracellular exosome from a different study (B-cell exosomes). The same concerns about plasma protein contamination apply. Reason: This is the same issue as the previous exosomal annotation but from a different study. PMID:20458337 analyzed "MHC class II-associated proteins in B-cell exosomes." While ITIH2 was detected, as an abundant plasma protein, it likely represents contamination rather than specific functional association with B-cell exosomes. The core functions of ITIH2 relate to ECM stabilization and hyaluronan binding, not exosome biology. Keep as non-core. Supporting Evidence: file:human/ITIH2/ITIH2-deep-research-perplexity-lite.md ITIH2 is a secreted plasma protein found in the extracellular space PMID:20458337 2010 May 11. MHC class II-associated proteins in B-cell exosomes and potential functional implications for exosome biogenesis. |
| GO:0005576 extracellular region | NAS PMID:14718574 The human plasma proteome: a nonredundant list developed by ... | ACCEPT | Summary: This NAS annotation is based on the plasma proteome study. This is a duplicate of the earlier GO:0005576 annotation but with experimental evidence (NAS) rather than IEA. Reason: This annotation is correct and supported by direct experimental evidence. PMID:14718574 identified ITIH2 in "The human plasma proteome" through multiple methodologies. This provides stronger evidence than the IEA annotation for the same term. ITIH2 is indeed located in the extracellular region (plasma and ECM). While this term is broad, it is accurate and well-supported by this proteomics study. Supporting Evidence: PMID:14718574 The human plasma proteome: a nonredundant list developed by combination of four separate sources |
| GO:0004866 endopeptidase inhibitor activity | TAS PMID:2476436 Analysis of inter-alpha-trypsin inhibitor and a novel trypsi... | REMOVE | Summary: This TAS annotation is from a traceable author statement in PMID:2476436. However, like the IEA annotations for protease inhibitor activity, this incorrectly attributes the inhibitory activity of the complex to the ITIH2 heavy chain. Reason: While PMID:2476436 is a foundational paper on inter-alpha-inhibitor structure, the endopeptidase inhibitor activity resides in bikunin, not ITIH2. The paper explicitly states "Analysis of the proteins, the separated chains, and proteolytic derivatives thereof revealed that each protein contained a single, identical, trypsin-inhibitory chain of 30,000 Da" (bikunin) and "Inter-alpha-trypsin inhibitor contains noninhibitory heavy chains of 65,000 and 70,000 Da." This annotation conflates the complex's activity with ITIH2's individual function. ITIH2 is a structural component that does not possess intrinsic endopeptidase inhibitor activity. Supporting Evidence: PMID:2476436 Inter-alpha-trypsin inhibitor contains noninhibitory heavy chains of 65,000 and 70,000 Da, whereas pre-alpha-trypsin inhibitor contains a heavy chain of 90,000 Da |
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