Myeloperoxidase: a key regulator of neutrophil oxidant production.

Myeloperoxidase plays a fundamental role in oxidant production by neutrophils. This heme enzyme uses hydrogen peroxide and chloride to catalyze the production of hypochlorous acid, which is the major strong oxidant generated by neutrophils in appreciable amounts. In addition to chlorination, myeloperoxidase displays several other activities. It readily oxidizes thiocyanate to hypothiocyanite, converts a myriad of organic substrates to reactive free radicals, and hydroxylates aromatic compounds. Depending on the concentration of its competing substrates and the conditions of the local environment, myeloperoxidase could substantially affect oxidant production by neutrophils. Superoxide is undoubtedly a physiological substrate for myeloperoxidase. Its interactions with the enzyme are key factors in determining how neutrophils use superoxide to kill pathogens and promote inflammatory tissue damage. Superoxide modulates the chlorination and peroxidation activities of myeloperoxidase. It also reacts with the enzyme to form oxymyeloperoxidase which is catalytically active and hydroxylates phenolic substrates. Myeloperoxidase reacts rapidly with nitric oxide and peroxynitrite so that at sites of inflammation there is a strong possibility that these reactions will impact on oxidative damage caused by neutrophils. Under certain conditions, many substrates of myeloperoxidase act as inhibitors and regulate oxidant production by the enzyme. Given the numerous reactions of myeloperoxidase, all its activities should be considered when assessing the injurious oxidants produced by neutrophils.