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Protein AMBP

Domain

Inter-alpha-trypsin inhibitor light chain

The Kunitz domains 1 and 2 serve as protease inhibitor domains.

Function

Alpha-1-microglobulin

Antioxidant and tissue repair protein with reductase, heme-binding and radical-scavenging activities. Removes and protects against harmful oxidants and repairs macromolecules in intravascular and extravascular spaces and in intracellular compartments (PubMed:11877257, PubMed:15683711, PubMed:22096585, PubMed:23157686, PubMed:23642167, PubMed:25698971, PubMed:32092412, PubMed:32823731). Intravascularly, plays a regulatory role in red cell homeostasis by preventing heme- and reactive oxygen species-induced cell damage. Binds and degrades free heme to protect fetal and adult red blood cells from hemolysis (PubMed:11877257, PubMed:32092412). Reduces extracellular methemoglobin, a Fe3+ (ferric) form of hemoglobin that cannot bind oxygen, back to the Fe2+ (ferrous) form deoxyhemoglobin, which has oxygen-carrying potential (PubMed:15683711). Upon acute inflammation, inhibits oxidation of low-density lipoprotein particles by MPO and limits vascular damage (PubMed:25698971). Extravascularly, protects from oxidation products formed on extracellular matrix structures and cell membranes. Catalyzes the reduction of carbonyl groups on oxidized collagen fibers and preserves cellular and extracellular matrix ultrastructures (PubMed:22096585, PubMed:23642167). Importantly, counteracts the oxidative damage at blood-placenta interface, preventing leakage of free fetal hemoglobin into the maternal circulation (PubMed:21356557). Intracellularly, has a role in maintaining mitochondrial redox homeostasis. Bound to complex I of the respiratory chain of mitochondria, may scavenge free radicals and preserve mitochondrial ATP synthesis. Protects renal tubule epithelial cells from heme-induced oxidative damage to mitochondria (PubMed:23157686, PubMed:32823731). Reduces cytochrome c from Fe3+ (ferric) to the Fe2+ (ferrous) state through formation of superoxide anion radicals in the presence of ascorbate or NADH/NADPH electron donor cofactors, ascorbate being the preferred cofactor (PubMed:15683711). Has a chaperone role in facilitating the correct folding of bikunin in the endoplasmic reticulum compartment (By similarity).

Inter-alpha-trypsin inhibitor light chain

Kunitz-type serine protease inhibitor and structural component of extracellular matrix with a role in extracellular space remodeling and cell adhesion (PubMed:20463016, PubMed:25301953). Among others, has antiprotease activity toward kallikrein, a protease involved in airway inflammation; inhibits GZMK/granzyme, a granule-stored serine protease involved in NK and T cell cytotoxic responses; and inhibits PLG/plasmin, a protease required for activation of matrix metalloproteinases (PubMed:10480954, PubMed:15917224, PubMed:16873769). As part of I-alpha-I complex, provides for the heavy chains to be transferred from I-alpha-I complex to hyaluronan in the presence of TNFAIP6, in a dynamic process that releases free bikunin and remodels extracellular matrix proteoglycan structures. Free bikunin, but not its heavy chain-bound form, acts as potent protease inhibitor in airway secretions (PubMed:16873769). Part of hyaluronan-rich extracellular matrix that surrounds oocyte during cumulus oophorus expansion, an indispensable process for proper ovulation (By similarity). Also inhibits calcium oxalate crystallization (PubMed:7676539).

Trypstatin

Kunitz-type serine protease inhibitor. Has high catalytic efficiency for F10/blood coagulation factor Xa and may act as an anticoagulant by inhibiting prothrombin activation. Inhibits trypsin and mast cell CMA1/chymase and tryptase proteases.

Post-translational modifications

The precursor is proteolytically processed into separately functioning proteins.

Alpha-1-microglobulin

Proteolytically cleaved in the presence of oxyhemoglobin or MPO (PubMed:11877257, PubMed:25698971). The cleaved form t-alpha-1-microglobulin lacks the C-terminal tetrapeptide LIPR and is released from IgA-alpha-1-microglobulin complex as well as from free alpha-1-microglobulin when exposed to oxyhemoglobin or erythrocyte membranes. The cleavage of IgA-alpha-1-microglobulin complex is associated with the reduction of the covalent bond between IgA and alpha-1-microglobulin, yielding an intact IgA molecule (PubMed:11877257). The cleavage by MPO is associated with the transfer of heme group from MPO to t-alpha-1-microglobulin (PubMed:25698971). t-alpha-1-microglobulin has higher reductase activity when compared with full length protein (PubMed:15683711).

Alpha-1-microglobulin

3-hydroxykynurenine, an oxidized tryptophan metabolite that is common in biological fluids, reacts with Cys-53, Lys-111, Lys-137, and Lys-149 to form heterogeneous polycyclic chromophores including hydroxanthommatin. The reaction by alpha-1-microglobulin is autocatalytic; the human protein forms chromophore even when expressed in insect and bacterial cells. The chromophore can react with accessible cysteines forming non-reducible thioether cross-links with other molecules of alpha-1-microglobulin or with other proteins such as Ig alpha-1 chain C region 'Cys-352'.

Inter-alpha-trypsin inhibitor light chain

Heavy chains are interlinked with bikunin via a chondroitin 4-sulfate bridge to the C-terminal aspartate.

Inter-alpha-trypsin inhibitor light chain

Proteolytically cleaved by PRSS3 at Kunitz domain 2.

N-glycosylated. N-glycan heterogeneity at Asn-115: Hex5HexNAc4 (major), Hex6HexNAc5 (minor) and dHex1Hex6HexNAc5 (minor). N-glycan at Asn-250: Hex5HexNAc4.

O-glycosylated. O-linkage of the glycosaminoglycan, chondroitin sulfate, at Ser-215 allows cross-linking between the three polypeptide chains.

Sequence Similarities

In the N-terminal section; belongs to the calycin superfamily. Lipocalin family.

Tissue Specificity

Alpha-1-microglobulin

Expressed by the liver and secreted in plasma. Occurs in many physiological fluids including plasma, urine, and cerebrospinal fluid (PubMed:11877257). Expressed in epidermal keratinocytes, in dermis and epidermal-dermal junction (at protein level) (PubMed:22096585). Expressed in red blood cells (at protein level) (PubMed:32092412). Expressed in placenta (PubMed:21356557).

Inter-alpha-trypsin inhibitor light chain

Detected in placenta (at protein level) (PubMed:32337544). Detected in cerebrospinal fluid, plasma and urine (at protein level) (PubMed:25326458, PubMed:36213313, PubMed:37453717). Expressed in airway epithelium and submucosal gland (at protein level). Colocalizes with TNFAIP6 at the ciliary border. Present in bronchoalveolar lavage fluid (at protein level).

Cellular localization

Alternative names

HCP, ITIL, Protein AMBP, Protein HC, Bikunin

swissprot:P02760