Nitric oxide-derived oxidants (e.g., peroxynitrite) are believed to participate in antimicrobial activities as part of normal host defenses but also in oxidative tissue injury in inflammatory disorders. A similar role is ascribed to the heme enzyme myeloperoxidase (MPO), the most abundant protein of polymorphonuclear leukocytes, which are the terminal phagocytosing effector cells of the innate immune system. Concomitant production of peroxynitrite and release of millimolar MPO are characteristic events during phagocytosis. In order to understand the mode of interaction between MPO and peroxynitrite, we have performed a comprehensive stopped-flow investigation of the reaction between all physiological relevant redox intermediates of MPO and peroxynitrite. Both iron(III) MPO and iron(II) MPO are rapidly converted to compound II by peroxynitrite in monophasic reactions with calculated rate constants of (6.8 +/- 0.1) X 10(6) M-1 s(-1) and (1.3 +/- 0.2) x 10(6) M-1 s(-1), respectively (pH 7.0 and 25 degrees C). Besides these one- and two-electron reduction reactions of peroxynitrite, which produce nitrogen dioxide and nitrite, a one-electron oxidation to the oxoperoxonitrogen radical must occur in the fast monophasic transition of compound I to compound II mediated by peroxynitrite at pH 7.0 [(7.6 +/- 0. 1) x 10(6) M-1 S-1]. In addition, peroxynitrite induced a steady-state transition from compound III to compound II with a rate of (1.0 +/- 0.3) x 10(4) M-1 s(-1). Thus, the interconversion among the various oxidation states of MPO that is prompted by peroxynitrite is remarkable. Reaction mechanisms are proposed and the physiological relevance is discussed. (c) 2005 Elsevier Inc. All rights reserved.