FUNCTION: 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. MPO is an important marker for myeloid cells, from the promyelocyte stage and to the mature forms. CATALYTIC ACTIVITY: Donor + H2O2 = oxidized donor + 2 H2O. CATALYTIC ACTIVITY: Cl- + H2O2 = HOCl + 2 H2O. COFACTOR: Binds 1 calcium ion per heterodimer. COFACTOR: Binds 1 heme B (iron-protoporphyrin IX) group covalently per heterodimer. SUBUNIT: Tetramer of two light chains and two heavy chains. SUBCELLULAR LOCATION: Lysosome. ALTERNATIVE PRODUCTS: 3 named isoforms produced by alternative splicing. DISEASE: Defects in MPO are the cause of myeloperoxidase deficiency (MPD). MPD is an autosomal recessive defect that results in disseminated candidiasis. SIMILARITY: Belongs to the peroxidase family. XPO subfamily.