The direct effects of the neurohormone melatonin on reactive oxygen species
(ROS) were investigated. Melatonin was found to inhibit DMPO-O-2(-) format
ion 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 rea
ction of xanthine and xanthine oxidase. The reaction rate constant of melat
onin 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 eviden
ce for the formation of the melatonin indolyl cation radical that presumabl
y precedes conversion of melatonin to its stable N-1-acetyl-N-2-5-methoxyky
nuramine (AMK) metabolite following sequential reactions of melatonin with
O-2(.-) and (OH)-O-.. On the other hand, melatonin was capable of scavengin
g H2O2 in a dose-dependent manner with an IC50 = 0.5 +/- 0.02 mM. The react
ion 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)-depe
ndent 2,2,6,6-tetramethylpiperidine oxide (TEMPO) radical formation during
rose bengal photodynamic reaction. The results suggest that melatonin's ant
ioxidant properties, in part, may involve a direct effect on scavenging of
ROS. (C) 1998 Elsevier Science B.V. All rights reserved.