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. Homodimer; disulfide-linked. Each monomer consists of a light and a heavy chain. Belongs to the peroxidase family. XPO subfamily. 3 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.