Haptoglobin

Haptoglobin or HP is encoded by the HP gene in humans. Haptoglobin binds free hemoglobin (which is released from erythrocytes) in the blood plasma, therefore inhibits the Hb oxidative activity. The complex that contains Haptoglobin and hemoglobin will then be eliminated mostly through the spleen.

Haptoglogin participates in intestinal permeability, permitting intercellular tight junction disassembly, and controlling the equilibrium among tolerance and immunity to non-self antigens. Haptoglobin has antibacterial activity and takes part in modulating many aspects of the acute phase response. Haptoglobin is processed to yield both alpha and beta chains, which later unite as a tetramer to produce haptoglobin. Haptoglobin binds free hemoglobin (Hb) secreted from erythrocytes In blood plasma with high affinity and thus inhibits its oxidative activity. Further, the haptoglobin-hemoglobin complex is being removed by the reticuloendothelial system.

Haptoglobin is a glycoprotein which is synthesized in the liver and circulates in the blood. Haptoglobin is produced typically by hepatocytes but also by other tissues: e.g. skin, lung, and kidney. It is a positive acute phase protein that binds free hemoglobin and removes it from the circulation to prevent kidney injury, and iron loss following hemolysis. The haptoglobin-hemoglobin complex is subsequently removed by the reticuloendothelial system (generally the spleen). As the reticuloendothelial system removes the haptoglobin-hemoglobin complex from the body, haptoglobin levels are reduced in hemolytic anaemias. In the course of binding hemoglobin, haptoglobin sequesters the iron inside hemoglobin, preventing iron-utilizing bacteria from benefitting from hemolysis.
Haptoglobin consists of two A- and two B-chains, connected by disulfide bonds. Three major haptoglobin phenotypes are known to exist (Hp 1-1, Hp 2-1, and Hp 2-2). Hp 1-1 is biologically the most effective in binding free hemoglobin and suppressing inflammatory responses associated with free hemoglobin. Hp 2-2 is biologically the least active, and Hp 2-1 is moderately active. Haptoglobin’s molecular mass ranges from 8-200 kDa.
Reduced levels can be seen in haemolysis and impaired liver function. High levels are a marker for acute or chronic inflammation. Ahaptoglobinemia or hypohaptoglobinemia are caused by mutations in the haptoglobin gene and/or its regulatory regions. Haptoglobin is also linked to diabetic nephropathy, the incidence of coronary artery disease in type 1 diabetes, Crohn's disease, inflammatory disease behavior, primary sclerosing cholangitis, susceptibility to idiopathic Parkinson's disease, and a reduced incidence of Plasmodium falciparum malaria.

Haptoglobin-Related Protein also known as HPR, is a primate-specific plasma protein related with apolipoprotein L-I (apoL-I)-containing high-density lipoprotein (HDL) particles which shown to be a part of the innate immune defense. HPR-bound hemoglobin might contribute to the biologic activity of the circulating apoL-I/Hprcontaining HDL particles. HPR is a clinically significant forecaster of recurrence of breast cancer.