Mechanism of reaction of melatonin with human myeloperoxidase

Recently, it was suggested that melatonin (N-acetyl-5-methoxytryptamine) is oxidized by activated neutrophils in a reaction most probably involving my eloperoxidase (Biochem. Biophys. Res. Commun. (2000) 279, 657-662). Myelope roxidase (MPO) is the most abundant protein of neutrophils and is involved in killing invading pathogens. To clarify if melatonin is a substrate of MP O, we investigated the oxidation of melatonin by its redox intermediates co mpounds I and II using transient-state spectral and kinetic measurements at 25 degreesC. Spectral and kinetic analysis revealed that both compound I a nd compound II oxidize melatonin via one-electron processes. The second-ord er 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), respecti vely. The rates for the one-electron reduction of compound II back to the f erric 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 fo r 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 c yanide. Melatonin can markedly inhibit the chlorinating activity of MPO at both pH 7 and pH 5. The implication of these findings in the activated neut rophil is discussed. (C) 2001 Academic Press.