FUNCTION As a result of hemolysis, hemoglobin is found to accumulate in the kidney and is secreted in the urine. Haptoglobin captures, and combines with free plasma hemoglobin to allow hepatic recycling of heme iron and to prevent kidney damage. Haptoglobin also acts as an antioxidant, has antibacterial activity, and plays a role in modulating many aspects of the acute phase response. Hemoglobin/haptoglobin complexes are rapidly cleared by the macrophage CD163 scavenger receptor expressed on the surface of liver Kupfer cells through an endocytic lysosomal degradation pathway.FUNCTION The uncleaved form of allele alpha-2 (2-2), known as zonulin, plays a role in intestinal permeability, allowing intercellular tight junction disassembly, and controlling the equilibrium between tolerance and immunity to non-self antigens.SUBUNIT Tetramer of two alpha and two beta chains; disulfide-linked (PubMed:4573324, PubMed:6997877). The hemoglobin/haptoglobin complex is composed of a haptoglobin dimer bound to two hemoglobin alpha-beta dimers (By similarity). Interacts with CD163 (By similarity). Interacts with ERGIC3 (PubMed:31142615).INTERACTION Expressed by the liver and secreted in plasma.POLYMORPHISM In human populations there are two major allelic forms, alpha-1 (1-1) with 83 residues and alpha-2 (2-2) with 142 residues. These alleles determine 3 possible genotypes, homozygous (1-1 or 2-2) and heterozygous (2-1), and 3 major phenotypes HP*1F/HP*1S and HP*2FS. The two main alleles of HP*1 are called HP*1F (fast) and HP*1S (slow). The alleles exhibit different oligomerization properties. In healthy males, but not in females, the Hp 2-2 phenotype is associated with higher serum iron, decreased antimicrobial and antioxidant capability, and less efficient clearance from the circulation, than Hp 1-1 and 2-1. The sequence displayed in this entry corresponds to allele alpha-2 (2-2).DISEASE The disease is caused by variants affecting the gene represented in this entry.SIMILARITY Belongs to the peptidase S1 family.CAUTION Although homologous to serine proteases, it has lost all essential catalytic residues and has no enzymatic activity.ONLINE INFORMATION Haptoglobin entry