Haptoglobin

General Information

Haptoglobin (frequently abbreviated as Hp) is a protein in the blood plasma that binds free hemoglobin 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). For this reason, haptoglobin levels will be decreased in haemolytic anaemias. Causes of haemolysis include transfusion reactions, certain drugs, and/or mechanical breakage, such as may be seen with some artificial heart valves. The destruction may be mild or severe, acute or chronic, and it can lead to haemolytic anaemia. Hp also , prevents bacteria from using the iron present in hemoglobin to grow, regulates the activity of many cell types of the immune system, and is an extracellular chaperone. It is produced mostly by hepatocytes but also by other tissues: e.g. skin, lung, and kidney. Liver disease may also result in low haptoglobin levels as liver damage may reduce both the production of haptoglobin and the removal of the haptoglobin-haemoglobin complexes. Hp in its simplest form cosists of two ?- and two ?-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 phenotypes of Hp, therefore are found in humans: Hp1-1, Hp2-1, and Hp2-2. Hp of different phenotypes have been shown to bind hemoglobin with different affinities, with Hp2-2 being the weakest binder.