The reaction of myeloperoxidase compound I (MPO-I) with chloride ion is widely assumed to produce the bacterial killing agent after phagocytosis. Two values of the rate constant for this important reaction have been published previously: 4.7 Â 10 6 m 21´s21 measured at 25 8C [Marquez, L.A. and Dunford, H.B. (1995) . The present paper is the result of a collaboration of the two groups to resolve the discrepancy in the rate constants. It was found that the rate constant for the reaction of compound I, generated from myeloperoxidase (MPO) and excess hydrogen peroxide with chloride, decreased with increasing chloride concentration. The rate constant published in 1995 was measured over a lower chloride concentration range; the 1998 rate constant at a higher range. Therefore the observed conversion of compound I to native enzyme in the presence of hydrogen peroxide and chloride ion cannot be attributed solely to the single elementary reaction MPO-I 1 Cl 2 3 MPO 1 HOCl. The simplest mechanism for the overall reaction which fit the experimental data is the following:where MPO-I±Cl 2 is a chlorinating intermediate. We can now say that the 1995 rate constant is k 2 and the corresponding reaction is rate-controlling at low [Cl 2 ]. At high [Cl 2 ], the reaction with rate constant k 3 is rate controlling. The 1998 rate constant for high [Cl 2 ] is a composite rate constant, approximated by k 2 k 3 /k 22 . Values of k 1 and k 21 are known from the literature. Results of this study yielded k 2 2.2 Â 10 6 m 21´s21 , k 22 1.9 Â 10 5 s 21 and k 3 5.2 Â 10 4 s 21 . Essentially identical results were obtained using human myeloperoxidase and beef spleen myeloperoxidase.

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The oxidation of serotonin (5-hydroxytryptamine) by the myeloperoxidase intermediates compounds I and II was investigated by using transient-state spectral and kinetic measurements at 25.0 +/- 0.1 degrees C. Rapid scan spectra demonstrated that both compound I and compound II oxidize serotonin via one-electron processes. Rate constants for these reactions were determined using both sequential-mixing and single-mixing stopped-flow techniques. The second order rate constant obtained for the one-electron reduction of compound I to compound II by serotonin is (1.7 +/- 0.1) x 10(7) M(-1) x s(-1), and that for compound II reduction to native enzyme is (1.4 +/- 0.1) x 10(6) M(-1) x s(-1) at pH 7.0. The maximum pH of the compound I reaction with serotonin occurs in the pH range 7.0-7.5. At neutral pH, the rate constant for myeloperoxidase compound I reacting with serotonin is an order of magnitude larger than for its reaction with chloride, (2.2 +/- 0.2) x 10(6) M(-1) x s(-1). A direct competition of serotonin with chloride for myeloperoxidase compound I oxidation was observed. Our results suggest that serotonin may have a role to protect lipoproteins from oxidation and to prevent enzymes from inactivation caused by the potent oxidants HOCl and active oxygen species.

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