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Haptoglobin
Identifiers
Symbols HP; MGC111141; hp2-alpha
External IDs OMIM140100 GeneCards: HP Gene
RNA expression pattern
PBB GE HP 206697 s at tn.png
PBB GE HP 208470 s at tn.png
More reference expression data
Orthologs
Species Human Mouse
Entrez 3240 n/a
Ensembl ENSG00000197711 n/a
UniProt P00738 n/a
RefSeq (mRNA) NM_005143 n/a
RefSeq (protein) NP_005134 n/a
Location (UCSC) Chr 16:
70.65 - 70.65 Mb
n/a
PubMed search [1] n/a

Haptoglobin (abbreviated as Hp) is a protein that in humans is encoded by the HP gene.[1][2] In blood plasma, haptoglobin binds free hemoglobin (Hb) released from erythrocytes with high affinity and thereby inhibits its oxidative activity. The haptoglobin-hemoglobin complex will then be removed by the reticuloendothelial system (mostly the spleen). In clinical settings, the haptoglobin assay is used to screen for and monitor intravascular hemolytic anemia. In intravascular hemolysis free hemoglobin will be released into circulation and hence haptoglobulin will bind the Hb. This causes a decline in Hp levels. Conversely, in extravascular hemolysis the reticuloendothelial system, especially splenic monocytes, phagocytose the erythrocytes and hemoglobin is not released into circulation and hence haptoglobulin levels are normal.

Contents

Function

This gene encodes a preproprotein that is processed to yield both alpha and beta chains, which subsequently combine as a tetramer to produce haptoglobin. Haptoglobin functions to bind free plasma hemoglobin, which allows degradative enzymes to gain access to the hemoglobin while at the same time preventing loss of iron through the kidneys and protecting the kidneys from damage by hemoglobin.[3]

Synthesis

Haptoglobin is produced mostly by hepatocytes but also by other tissues: e.g., skin, lung, and kidney. In addition, the haptoglobin gene is expressed in murine and human adipose tissue.[4]

Structure

Haptoglobin, in its simplest form, consists of two α- and two β-chains, connected by disulfide bridges. The chains originate from a common precursor protein, which is proteolytically cleaved during protein synthesis.

Hp exists in two allelic forms in the human population, so-called Hp1 and Hp2, the latter one having arisen due to the partial duplication of Hp1 gene. Three genotypes of Hp, therefore, are found in humans: Hp1-1, Hp2-1, and Hp2-2. Hp of different genotypes have been shown to bind hemoglobin with different affinities, with Hp2-2 being the weakest binder.

In other species

Hp has been found in all mammals studied so far, some birds, e.g., cormorant and ostrich but also, in its simpler form, in bony fish, e.g., zebrafish. It is interesting to note that Hp is absent in at least some amphibians (Xenopus) and neognathous birds (chicken and goose).

Clinical significance

Mutations in this gene and/or its regulatory regions cause ahaptoglobinemia or hypohaptoglobinemia. This gene has also been linked to diabetic nephropathy,[5] the incidence of coronary artery disease in type 1 diabetes,[6] Crohn's disease,[7] inflammatory disease behavior, primary sclerosing cholangitis, susceptibility to idiopathic Parkinson's disease,[8] and a reduced incidence of Plasmodium falciparum malaria.[9]

Since the reticuloendothelial system will remove the haptoglobin-hemoglobin complex from the body, haptoglobin levels will be decreased in hemolytic anemias. In the process of binding hemoglobin, haptoglobin sequesters the iron within hemoglobin, preventing iron-utilizing bacteria from benefiting from hemolysis. It is theorized that, because of this, haptoglobin has evolved into an acute-phase protein. HP has a protective influence on the hemolytic kidney.[10][11]

Some studies associate the HP with the risk of developing schizophrenia.[12]

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Test protocol

Haptoglobin is ordered whenever a patient exhibits symptoms of anemia, such as pallor, fatigue, or shortness of breath, along with physical signs of hemolysis, such as jaundice or dark-colored urine. The test is also commonly ordered as a hemolytic anemia battery, which also includes a reticulocyte count and a peripheral blood smear. It can also be ordered along with a Direct Antiglobulin Test when a patient is suspected of having a transfusion reaction or symptoms of autoimmune hemolytic anemia. Also, it may be ordered in conjunction with a bilirubin.

Interpretation

A decrease in haptoglobin can support a diagnosis of hemolytic anemia, especially when correlated with a decreased RBC count, Hemoglobin, and Hematocrit, and also an increased reticulocyte count.

If the reticulocyte count is increased, but the haptoglobin level is normal, this may indicate that cellular destruction is occurring in the spleen and liver, which may indicate a drug-induced hemolysis, or a red cell dysplasia. The spleen and liver recognize an error in the red cells (either Drug coating the red cell membrane or a dysfunctional red cell membrane), and destroy the cell. This type of destruction does not release hemoglobin into the peripheral blood, so the haptoglobin cannot bind to it. Thus, the haptoglobin will stay normal.

If there are symptoms of anemia but both the reticulocyte count and the haptoglobin level are normal, the anemia is most likely not due to hemolysis, but instead some other error in cellular production, such as aplastic anemia

Haptoglobin levels that are decreased but do not accompany signs of anemia may indicate liver damage, as the liver is not producing enough haptoglobin to begin with.

As haptoglobin is indeed an acute-phase protein, any inflammatory process (infection, extreme stress, burns, major crush injury, allergy, etc.) may increase the levels of plasma haptoglobin.

References

  1. ^ Dobryszycka W (September 1997). "Biological functions of haptoglobin--new pieces to an old puzzle". Eur J Clin Chem Clin Biochem 35 (9): 647–54. PMID 9352226.  
  2. ^ Wassell J (2000). "Haptoglobin: function and polymorphism". Clin. Lab. 46 (11-12): 547–52. PMID 11109501.  
  3. ^ "Entrez Gene: HP". http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=3240.  
  4. ^ Trayhurn P, Wood IS (September 2004). "Adipokines: inflammation and the pleiotropic role of white adipose tissue". Br. J. Nutr. 92 (3): 347–55. doi:10.1079/BJN20041213. PMID 15469638.  
  5. ^ Asleh R, Levy AP (2005). "In vivo and in vitro studies establishing haptoglobin as a major susceptibility gene for diabetic vascular disease". Vasc Health Risk Manag 1 (1): 19–28. doi:10.2147/vhrm.1.1.19.58930. PMID 17319095.  
  6. ^ Sadrzadeh SM, Bozorgmehr J (June 2004). "Haptoglobin phenotypes in health and disorders". Am. J. Clin. Pathol. 121 Suppl: S97–104. PMID 15298155.  
  7. ^ Papp M, Lakatos PL, Palatka K, Foldi I, Udvardy M, Harsfalvi J, Tornai I, Vitalis Z, Dinya T, Kovacs A, Molnar T, Demeter P, Papp J, Lakatos L, Altorjay I (May 2007). "Haptoglobin polymorphisms are associated with Crohn's disease, disease behavior, and extraintestinal manifestations in Hungarian patients". Dig. Dis. Sci. 52 (5): 1279–84. doi:10.1007/s10620-006-9615-1. PMID 17357835.  
  8. ^ Costa-Mallen P, Checkoway H, Zabeti A, Edenfield MJ, Swanson PD, Longstreth WT, Franklin GM, Smith-Weller T, Sadrzadeh SM (March 2008). "The functional polymorphism of the hemoglobin-binding protein haptoglobin influences susceptibility to idiopathic Parkinson's disease". Am. J. Med. Genet. B Neuropsychiatr. Genet. 147B (2): 216–22. doi:10.1002/ajmg.b.30593. PMID 17918239.  
  9. ^ Prentice AM, Ghattas H, Doherty C, Cox SE (December 2007). "Iron metabolism and malaria". Food Nutr Bull 28 (4 Suppl): S524–39. PMID 18297891.  
  10. ^ Pintera J (1968). "The protective influence of haptoglobin on hemoglobinuric kidney. I. Biochemical and macroscopic observations". Folia Haematol. Int. Mag. Klin. Morphol. Blutforsch. 90 (1): 82–91. PMID 4176393.  
  11. ^ Miederer SE, Hotz J (December 1969). "[Pathogenesis of kidney hemolysis]" (in German). Bruns Beitr Klin Chir 217 (7): 661–5. PMID 5404273.  
  12. ^ Gene Overview of All Published Schizophrenia-Association Studies for HP - SzGene database at Schizophrenia Research Forum.

Further reading

  • Graversen JH, Madsen M, Moestrup SK (2002). "CD163: a signal receptor scavenging haptoglobin-hemoglobin complexes from plasma.". Int. J. Biochem. Cell Biol. 34 (4): 309–14. doi:10.1016/S1357-2725(01)00144-3. PMID 11854028.  
  • Madsen M, Graversen JH, Moestrup SK (2002). "Haptoglobin and CD163: captor and receptor gating hemoglobin to macrophage lysosomes.". Redox Rep. 6 (6): 386–8. doi:10.1179/135100001101536490. PMID 11865982.  
  • Erickson LM, Kim HS, Maeda N (1993). "Junctions between genes in the haptoglobin gene cluster of primates.". Genomics 14 (4): 948–58. doi:10.1016/S0888-7543(05)80116-8. PMID 1478675.  
  • Maeda N (1985). "Nucleotide sequence of the haptoglobin and haptoglobin-related gene pair. The haptoglobin-related gene contains a retrovirus-like element.". J. Biol. Chem. 260 (11): 6698–709. PMID 2987228.  
  • Simmers RN, Stupans I, Sutherland GR (1986). "Localization of the human haptoglobin genes distal to the fragile site at 16q22 using in situ hybridization.". Cytogenet. Cell Genet. 41 (1): 38–41. doi:10.1159/000132193. PMID 3455911.  
  • van der Straten A, Falque JC, Loriau R, et al. (1986). "Expression of cloned human haptoglobin and alpha 1-antitrypsin complementary DNAs in Saccharomyces cerevisiae.". DNA 5 (2): 129–36. PMID 3519135.  
  • Bensi G, Raugei G, Klefenz H, Cortese R (1985). "Structure and expression of the human haptoglobin locus.". Embo J. 4 (1): 119–26. PMID 4018023.  
  • Malchy B, Dixon GH (1973). "Studies on the interchain disulfides of human haptoglobins.". Can. J. Biochem. 51 (3): 249–64. PMID 4573324.  
  • Raugei G, Bensi G, Colantuoni V, et al. (1983). "Sequence of human haptoglobin cDNA: evidence that the alpha and beta subunits are coded by the same mRNA.". Nucleic Acids Res. 11 (17): 5811–9. doi:10.1093/nar/11.17.5811. PMID 6310515.  
  • Yang F, Brune JL, Baldwin WD, et al. (1983). "Identification and characterization of human haptoglobin cDNA.". Proc. Natl. Acad. Sci. U.S.A. 80 (19): 5875–9. doi:10.1073/pnas.80.19.5875. PMID 6310599.  
  • Maeda N, Yang F, Barnett DR, et al. (1984). "Duplication within the haptoglobin Hp2 gene.". Nature 309 (5964): 131–5. doi:10.1038/309131a0. PMID 6325933.  
  • Brune JL, Yang F, Barnett DR, Bowman BH (1984). "Evolution of haptoglobin: comparison of complementary DNA encoding Hp alpha 1S and Hp alpha 2FS.". Nucleic Acids Res. 12 (11): 4531–8. PMID 6330675.  
  • van der Straten A, Herzog A, Cabezón T, Bollen A (1984). "Characterization of human haptoglobin cDNAs coding for alpha 2FS beta and alpha 1S beta variants.". FEBS Lett. 168 (1): 103–7. PMID 6546723.  
  • vander Straten A, Herzog A, Jacobs P, et al. (1984). "Molecular cloning of human haptoglobin cDNA: evidence for a single mRNA coding for alpha 2 and beta chains.". Embo J. 2 (6): 1003–7. PMID 6688992.  
  • Kurosky A, Barnett DR, Lee TH, et al. (1980). "Covalent structure of human haptoglobin: a serine protease homolog.". Proc. Natl. Acad. Sci. U.S.A. 77 (6): 3388–92. doi:10.1073/pnas.77.6.3388. PMID 6997877.  
  • Eaton JW, Brandt P, Mahoney JR, Lee JT (1982). "Haptoglobin: a natural bacteriostat.". Science 215 (4533): 691–3. doi:10.1126/science.7036344. PMID 7036344.  
  • Kazim AL, Atassi MZ (1980). "Haemoglobin binding with haptoglobin. Unequivocal demonstration that the beta-chains of human haemoglobin bind to haptoglobin.". Biochem. J. 185 (1): 285–7. PMID 7378053.  
  • Hillier LD, Lennon G, Becker M, et al. (1997). "Generation and analysis of 280,000 human expressed sequence tags.". Genome Res. 6 (9): 807–28. doi:10.1101/gr.6.9.807. PMID 8889549.  
  • Tabak S, Lev A, Valansi C, et al. (1997). "Transcriptionally active haptoglobin-related (Hpr) gene in hepatoma G2 and leukemia molt-4 cells.". DNA Cell Biol. 15 (11): 1001–7. doi:10.1089/dna.1996.15.1001. PMID 8945641.  
  • Koda Y, Soejima M, Yoshioka N, Kimura H (1998). "The haptoglobin-gene deletion responsible for anhaptoglobinemia.". Am. J. Hum. Genet. 62 (2): 245–52. doi:10.1086/301701. PMID 9463309.  

External links

This article incorporates text from the United States National Library of Medicine, which is in the public domain.


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