Direct conversion of ferrous myeloperoxidase to compound II by hydrogen peroxide: an anaerobic stopped-flow study

Myeloperoxidase (MPO) is one of the essential components of the antimicrobial systems of polymorphonuclear neutrophils. It is unique in having a globin-like standard reduction potential of the ferric/ferrous couple. Here, it is shown that ferrous MPO heterolytically cleaves hydrogen peroxide forming water and oxyferryl MPO (compound II). The two-electron oxidation reaction follows second-order kinetics with the apparent bimolecular rate constant being (6.8+/-0.6)x10(4)M(-1)s(-1) at pH 7.0. After depletion of (micromolar) H(2)O(2) compound II slowly decays to ferric MPO, whereas upon addition of millimolar H(2)O(2) to ferrous MPO, compound III (oxyperoxidase) is formed in a sequence of two reactions involving compound II formation and its direct reaction with H(2)O(2), which also follows second-order kinetics [(78+/-2)M(-1)s(-1) at pH 7.0]. It is discussed how these reactions contribute to the interconversion of compound II and compound III and could explain the catalase activity of MPO.