Scavenging of reactive oxygen species by melatonin

The direct effects of the neurohormone melatonin on reactive oxygen species (ROS) were investigated. Melatonin was found to inhibit DMPO-O-2(-) formation in a dose-dependent manner. At the level of 1.7 +/- 0.07 mM, melatonin caused 50% inhibition of EPR signal intensity of DMPO-O-2(-) during the reaction of xanthine and xanthine oxidase. The reaction rate constant of melatonin with O-2(.-) was found to be 1.25 +/- 0.07 x 10(3) M-1 s(-1). However, melatonin (up to 1.2 mM) did not exhibit significant effect toward (OH)-O-. radical, produced by the Fenton reaction. In addition, we found no evidence for the formation of the melatonin indolyl cation radical that presumably precedes conversion of melatonin to its stable N-1-acetyl-N-2-5-methoxykynuramine (AMK) metabolite following sequential reactions of melatonin with O-2(.-) and (OH)-O-.. On the other hand, melatonin was capable of scavenging H2O2 in a dose-dependent manner with an IC50 = 0.5 +/- 0.02 mM. The reaction rate constant of melatonin with H2O2 was found to be 2.52 +/- 0.19 x 10(5) M-1 s(-1). Furthermore, melatonin was also found to inhibit O-1(2)-dependent 2,2,6,6-tetramethylpiperidine oxide (TEMPO) radical formation during rose bengal photodynamic reaction. The results suggest that melatonin's antioxidant properties, in part, may involve a direct effect on scavenging of ROS. (C) 1998 Elsevier Science B.V. All rights reserved.