Reactions of superoxide with myeloperoxidase and its products.

Myeloperoxidase (MPO) uses hydrogen peroxide to oxidize chloride to hypochlorous acid. It also converts numerous substrates to reactive free radicals. When released by neutrophils, the enzyme operates in the presence of a flux of superoxide. We show that superoxide has a profound influence on oxidative reactions catalysed by MPO. It reacts directly with the enzyme to modulate production of hypochlorous acid. Within neutrophil phagosomes, where MPO functions to kill micro-organisms, it may be the preferred substrate for the enzyme. Superoxide also reacts rapidly with radicals generated by MPO, e.g. from tyrosine and tyrosyl peptides. Initial products are organic peroxides. These species are likely to be toxic and contribute to the pathophysiological actions of MPO.