Part of the host defense system of polymorphonuclear leukocytes. It is responsible for microbicidal activity against a wide range of organisms. In the stimulated PMN, MPO catalyzes the production of hypohalous acids, primarily hypochlorous acid in physiologic situations, and other toxic intermediates that greatly enhance PMN microbicidal activity (PubMed:9922160). Mediates the proteolytic cleavage of alpha-1-microglobulin to form t-alpha-1-microglobulin, which potently inhibits oxidation of low-density lipoprotein particles and limits vascular damage (PubMed:25698971). chloride + H(+) + H2O2 = H2O + hypochlorous acid Binds 1 Ca(2+) ion per monomer. Binds 1 heme b (iron(II)-protoporphyrin IX) group covalently per monomer. Homodimer; disulfide-linked. Each monomer consists of a light and a heavy chain. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peroxidase family. XPO subfamily. MPO mutation db Myeloperoxidase entry