Kinetic studies were performed with microperoxidase-8 (Fe(III)MP-8), the proteolytic breakdown product of horse heart cytochrome c containing an octapepticle linked to an iron protoporphyrin IX. Mn(III) was substituted for Fe(Ill) in Mn(III)MP-8. The mechanism of formation of the reactive metal-oxo and metal-hydroperoxo intermediates of M(III)MP-8 upon reaction of H2O2 with Fe(III)MP-8 and Mn(III)MP-8 was investigated by rapid-scan stopped-flow spectroscopy and transient EPR. Two steps (k(obs1) and k(obs2)) were observed and analyzed for the reaction of hydrogen peroxide with both catalysts. The plots of k(obs1) as function of [H2O2] at pH 8.0 and pH 9.1 for Fe(III)MP-8, and at pH 10.2 and pH 10.9 for Mn(III)MP-8, exhibit saturation kinetics, which reveal the accumulation of an intermediate. Double reciprocal plots of 1/k(obs1) as function of 1/[H2O2] at different pH values reveal a competitive effect of protons in the oxidation of M(III)MP-8. This effect of protons is confirmed by the linear dependence of 1/k(obs1) on [H+] showing that k(obs1) increases with the pH. The UV-visible spectra of the intermediates formed at the end of the first step (k(obs1)) exhibit a spectrum characteristic of a high-valent metal-oxo intermediate for both catalysts. Transient EPR of Mn(III)MP-8 incubated with an excess of H2O2, at pH 11.5, shows the detection of a free radical signal at g approximate to 2 and of a resonance at g approximate to 4 characteristic of a Mn(IV) (S = 3/2) species. On the basis of these results, the following mechanism is proposed: (i) M(III)MP-8-OH2 is deprotonated to M(III)MP-8-OH in a rapid preequilibrium step, with a pK(a) = 9.2 +/- 0.9 for Fe(III)MP-8 and a pK(a) = 11.2 +/- 0.3 for Mn(III)-MP-8; (ii) M(III)MP-8-OH reacts with H2O2 to form Compound 0, M(III)MP8-OOH, with a second-order rate constant k(1) = (1.3 +/- 0.6) x 10(6) M-1.s(-1) for Fe(III)MP-8 and k(1) = (1.6 +/- 0.9) x 10(5) M-1.S-1 for Mn(III)-MP-8; (iii) this metal-hydroperoxo intermediate is subsequently converted to a high-valent metal-oxo species, M(IV)MP-8=O, with a free radical on the peptide (R.+). The first-order rate constants for the cleavage of the hydroperoxo group are k(2) = 165 +/- 8 s(-1) for Fe(III)MP-8 and k(2) = 145 +/- 7 s(-1) for Mn(III)MP-8; and (iv) the proposed M(IV)MP-8=O(R.+) intermediate slowly decays (k(obs2)) with a rate constant of k(obs2) = 13.1 +/- 1.1 s(-1) for Fe(III)MP-8 and k(obs2) = 5.2 +/- 1.2 s(-1) for Mn(III)MP-8. The results show that Compound 0 is formed prior to what is analyzed as a high-valent metal-oxo peptide radical intermediate.