Kinetics of oxidation of serotonin by myeloperoxidase compounds I and II.

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 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) + 10(7) M(-1) s(-1), and that for compound II reduction to native enzyme is (1.4 +/- 0.1) + 10(6) M(-1) 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) + 10(6) M(-1) 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.