Hemopexin (or haemopexin; HPX), also known as beta-1B-glycoprotein is a protein that in humans is encoded by the HPXgene[1][2][3] and belongs to hemopexin family of proteins.[4] Hemoglobin and its scavenger protein hemopexin (Hx) associate with HDL[expand acronym] and influence the inflammatory properties of HDL. In addition it can also be said that HDL from Hx-null mice is proinflammatory. Moreover, hemopexin deficiency is associated with various other inflammatory diseases such as septic shock and experimental autoimmune encephalomyelitis.[5]
Takahashi et al. (1985) determined that human plasma beta-glycoprotein hemopexin consists of a single polypeptide chain of 439 amino acids residues with six intrachain disulfide bridges and has a molecular mass of approximately 63 kD. The amino-terminal threonine residue is blocked by an O-linked galactosamineoligosaccharide, and the protein has five glucosamine oligosaccharides N-linked to the acceptor sequence Asn-X-Ser/Thr. The 18 tryptophan residues are arranged in four clusters, and 12 of the tryptophans are conserved in homologous positions. Computer-assisted analysis of the internal homology in amino acid sequence suggested duplication of an ancestral gene thus indicating that hemopexin consists of two similar halves.[6]
Altruda et al. (1988) demonstrated that the hemopexin gene spans approximately 12 kb and is interrupted by 9 exons. The demonstration shows direct correspondence between exons and the 10 repeating units in the protein. As the introns were not placed randomly; they fell in the center of the region of amino acid sequence homology in strikingly similar locations in 6 of the 10 units and in a symmetric position in each half of the coding sequence. From these observations, Altruda et al. (1988) concluded that the gene evolved through intron-mediated duplications of a primordial sequence to a 5-exon cluster.[7]
Mapping of hemopexin gene
Cai and Law (1986) prepared a cDNA clone for hemopexin, by Southern blot analysis of human/hamster hybrids containing different combinations of human chromosomes, assigned the hemopexin gene to human chromosome 11.
Law et al. (1988) assigned the hemopexin gene to 11p15.5-p15.4, the same location as that of the beta-globin gene complex by in situ hybridization.[8]
Function
Hemopexin binds heme with the highest affinity of any known protein. Its function is scavenging the heme released or lost by the turnover of heme proteins such as hemoglobin and thus protects the body from the oxidative damage that free heme can cause. In addition, hemopexin releases its bound ligand for internalisation upon interacting with a specific receptor situated on the surface of liver cells. This function of hemopexin is to preserve the body's iron.[9] Hemopexin, an acute phase protein, can downregulate the angiotensin (ang) II type 1 receptor (AT1-R) in vitro.[10]
Hx-dependent uptake of extracellular heme can lead to the deactivation of Bach1 repression which leads to the transcriptional activation of antioxidant heme oxygenase-1 gene. There are certain levels of circulating Hx which implicates in the prognosis for patients with septic shock. Therefore, it can also be said that, Hx therapy has been shown to be beneficial in cardiovascular disease, cerebral ischemic injury, and experimental autoimmune encephalomyelitis.[11]
Clinical significance
Its levels in serum reflect how much heme is present in the blood. Therefore, low hemopexin levels indicates that there has been significant degradation of heme containing compounds and hemopexin is made to scavenge any heme it can. Low hemopexin levels are one of the diagnostic features of an intravascular hemolytic anemia.[12]
Controversies
In past there have been reports showing from patients with sickle cell disease, spherocytosis, autoimmune hemolytic anemia, erythropoietic protoporphyria and pyruvate kinase deficiency which have been suggested that haptoglobin (Hp) depletion in plasma occurs prior to the decline of hemopexin (Hx) concentrations.[13]
Heme released during oxidation of Hb to met-Hb or from heme saturated hepatocytes is bound by albumin and rapidly transferred to Hx, the plasma protein with the highest binding affinity for heme. Hx is glycoprotein produced by the liver with a plasma concentration of 1–2 mg/ml.[13] Hx prevents heme's pro-oxidant and pro-inflammatory effects and it also promotes its detoxification, specifically when Hp concentrations are low or depleted in cases of severe or prolonged hemolysis. Hp and Hx, both are acute-phase proteins, induced during infection and inflammatory states to minimize tissue injury and facilitate tissue repair.The current review also suggests that the primary mechanisms by which Hp and Hx prevent heme toxicity prior to monocyte or macrophage clearance, it also critically evaluate the difference in genetic phenotype function and describe the rationale for exogenous Hp and Hx as therapeutic proteins.[14]
Mutations
Deletion of the hemopexin or heme oxygenase-2 gene can aggravate brain injury followed by stroma-free hemoglobin-induced intracerebral haemorrhage.[15]
Differential transcriptional pattern of hemopexin gene
The expression of the human hemopexin gene in different human tissues and cell lines was carried out by using the specific cDNA probe. From the results obtained it can be concluded that this gene is expressed in liver and, in lower amount, in hepatoma cell lines but not in kidney, spleen, placental cells, and in HeLa, fibroblast cell lines.By S1 mapping it can also said that the transcription initiation site in hepatic cells is 28 base pairs upstream from the AUG initiation codon of the hemopexin gene.[16]
↑Altruda F, Poli V, Restagno G, Silengo L (1988). "Structure of the human hemopexin gene and evidence for intron-mediated evolution". Journal of Molecular Evolution. 27 (2): 102–8. doi:10.1007/BF02138368. PMID2842511.
↑Tolosano E, Altruda F (April 2002). "Hemopexin: structure, function, and regulation". DNA and Cell Biology. 21 (4): 297–306. doi:10.1089/104454902753759717. PMID12042069.
↑Krikken JA, Lely AT, Bakker SJ, Borghuis T, Faas MM, van Goor H, Navis G, Bakker WW (March 2013). "Hemopexin activity is associated with angiotensin II responsiveness in humans". Journal of Hypertension. 31 (3): 537–41. doi:10.1097/HJH.0b013e32835c1727. PMID23254305.
↑Poli V, Altruda F, Silengo L (1986). "Differential transcriptional pattern of the hemopexin gene". The Italian Journal of Biochemistry. 35 (5): 355–60. PMID3026994.
Further reading
Piccard H, Van den Steen PE, Opdenakker G (April 2007). "Hemopexin domains as multifunctional liganding modules in matrix metalloproteinases and other proteins". Journal of Leukocyte Biology. 81 (4): 870–92. doi:10.1189/jlb.1006629. PMID17185359.
Morgan WT, Muller-Eberhard U, Lamola AA (January 1978). "Interaction of rabbit hemopexin with bilirubin". Biochimica et Biophysica Acta. 532 (1): 57–64. doi:10.1016/0005-2795(78)90447-6. PMID620056.
Liu HM, Atack JR, Rapoport SI (1989). "Immunohistochemical localization of intracellular plasma proteins in the human central nervous system". Acta Neuropathologica. 78 (1): 16–21. doi:10.1007/BF00687397. PMID2735186.
Smith A, Tatum FM, Muster P, Burch MK, Morgan WT (April 1988). "Importance of ligand-induced conformational changes in hemopexin for receptor-mediated heme transport". The Journal of Biological Chemistry. 263 (11): 5224–9. PMID2833500.
Altruda F, Poli V, Restagno G, Silengo L (1988). "Structure of the human hemopexin gene and evidence for intron-mediated evolution". Journal of Molecular Evolution. 27 (2): 102–8. doi:10.1007/BF02138368. PMID2842511.
Taketani S, Kohno H, Naitoh Y, Tokunaga R (June 1987). "Isolation of the hemopexin receptor from human placenta". The Journal of Biological Chemistry. 262 (18): 8668–71. PMID3036819.
Law ML, Cai GY, Hartz JA, Jones C, Kao FT (July 1988). "The hemopexin gene maps to the same location as the beta-globin gene cluster on human chromosome 11". Genomics. 3 (1): 48–52. doi:10.1016/0888-7543(88)90158-9. PMID3220477.
Morgan WT, Alam J, Deaciuc V, Muster P, Tatum FM, Smith A (June 1988). "Interaction of hemopexin with Sn-protoporphyrin IX, an inhibitor of heme oxygenase. Role for hemopexin in hepatic uptake of Sn-protoporphyrin IX and induction of mRNA for heme oxygenase". The Journal of Biological Chemistry. 263 (17): 8226–31. PMID3372522.
Frantíková V, Borvák J, Kluh I, Morávek L (December 1984). "Amino acid sequence of the N-terminal region of human hemopexin". FEBS Letters. 178 (2): 213–6. doi:10.1016/0014-5793(84)80603-1. PMID6510521.
Smith A, Alam J, Escriba PV, Morgan WT (April 1993). "Regulation of heme oxygenase and metallothionein gene expression by the heme analogs, cobalt-, and tin-protoporphyrin". The Journal of Biological Chemistry. 268 (10): 7365–71. PMID8463269.
Morris CM, Candy JM, Edwardson JA, Bloxham CA, Smith A (January 1993). "Evidence for the localization of haemopexin immunoreactivity in neurones in the human brain". Neuroscience Letters. 149 (2): 141–4. doi:10.1016/0304-3940(93)90756-B. PMID8474687.
Hrkal Z, Kuzelová K, Muller-Eberhard U, Stern R (March 1996). "Hyaluronan-binding properties of human serum hemopexin". FEBS Letters. 383 (1–2): 72–4. doi:10.1016/0014-5793(96)00225-6. PMID8612795.
Miller YI, Smith A, Morgan WT, Shaklai N (October 1996). "Role of hemopexin in protection of low-density lipoprotein against hemoglobin-induced oxidation". Biochemistry. 35 (40): 13112–7. doi:10.1021/bi960737u. PMID8855948.
Grinberg LN, O'Brien PJ, Hrkal Z (July 1999). "The effects of heme-binding proteins on the peroxidative and catalatic activities of hemin". Free Radical Biology & Medicine. 27 (1–2): 214–9. doi:10.1016/S0891-5849(99)00082-9. PMID10443938.
Nakajima S, Moriyama T, Hayashi H, Sakata I, Nakae Y, Takemura T (February 2000). "Hemopexin as a carrier protein of tumor-localizing Ga-metalloporphyrin-ATN-2". Cancer Letters. 149 (1–2): 221–6. doi:10.1016/S0304-3835(99)00367-5. PMID10737728.
Shipulina N, Smith A, Morgan WT (April 2000). "Heme binding by hemopexin: evidence for multiple modes of binding and functional implications". Journal of Protein Chemistry. 19 (3): 239–48. doi:10.1023/A:1007016105813. PMID10981817.