Reactions of myeloperoxidase. Ferric myeloperoxidase (MP³⁺) reacts with hydrogen peroxide to form the redox intermediate compound I, which oxidizes chloride or thiocyanate by a single 2-electron transfer to produce the respective hypohalous acids. Myeloperoxidase also oxidizes numerous organic substrates (RH) by two successive 1-electron transfers involving the enzyme intermediates compound I and compound II. Poor peroxidase substrates trap the enzyme as compound II and hypohalous acid production is inhibited unless superoxide is present to recycle the native enzyme. Superoxide can convert myeloperoxidase to compound III, which is turned over by a second reaction with superoxide. It has yet to be established whether the products of the latter reaction are compound I or MP³⁺ and hydrogen peroxide. Either way, the same net result is achieved.