Recently, it was suggested that melatonin (N-acetyl-5-methoxytryptamine) is oxidized by activated neutrophils in a reaction most probably involving myeloperoxidase (Biochem. Biophys. Res. Commun. (2000) 279, 657-662). Myeloperoxidase (MPO) is the most abundant protein of neutrophils and is involved in killing invading pathogens. To clarify if melatonin is a substrate of MPO, we investigated the oxidation of melatonin by its redox intermediates compounds I and II using transient-state spectral and kinetic measurements at 25 degreesC. Spectral and kinetic analysis revealed that both compound I and compound II oxidize melatonin via one-electron processes. The second-order rate constant measured for compound I reduction at pH 7 and pH 5 are (6.1 +/- 0.2) x 10(6) M-1 s(-1) and (1.0 +/- 0.08) x 10(7) M-1 s(-1), respectively. The rates for the one-electron reduction of compound II back to the ferric enzyme are (9.6 +/- 0.3) x 10(2) M-1 s(-1) (pH 7) and (2.2 +/- 0.1) x 10(3) M-1 s(-1) (pH 5), Thus, melatonin is a much better electron donor for compound I than for compound II, Steady-state experiments showed that the rate of oxidation of melatonin is dependent on the H2O2 concentration, is not affected by superoxide dismutase, and is quickly terminated by sodium cyanide. Melatonin can markedly inhibit the chlorinating activity of MPO at both pH 7 and pH 5. The implication of these findings in the activated neutrophil is discussed.