Myeloperoxidase (MPO) is a heterotetrameric protein consisting of two 60 kD heavy units and two 12 kD light units. A lysosomal enzyme, MPO is able to catalyze the production of hypochlorous acid, a potent microbicidal agent, from hydrogen peroxide and chloride anion during the neutrophil respiratory burst. MPO is a major enzyme involved in the inflammatory responses of polymorphonuclear leucocytes. MPO is localized to the azurophilic granules of mature granulocytes and monocytes and is also expressed in some acute myeloid leukemia cells.