title: Scavenging of reactive oxygen species by melatonin.
abstract: 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 O2- was found to be 1.25+/-0.07x103 M-1 s-1. However, melatonin (up to 1.2 mM) did not exhibit significant effect toward OH 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 N1-acetyl-N2-5-methoxykynuramine (AMK) metabolite following sequential reactions of melatonin with O2- and OH. 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+/-0.19x105 M-1 s-1. Furthermore, melatonin was also found to inhibit 1O2-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.
authors.affiliation: Pathology and Physiology Research Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, 1095 Willowdale Road, Morgantown, WV 26505-2888, USA. laz7@cdc.gov
authors.forename: L Y
authors.initials: LY
authors.lastname: Zang
authors.forename: G
authors.initials: G
authors.lastname: Cosma
authors.forename: H
authors.initials: H
authors.lastname: Gardner
authors.forename: V
authors.initials: V
authors.lastname: Vallyathan
mesh_terms: D000975:Antioxidants / Q000494:pharmacology*; D003497:Cyclic N-Oxides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D016166:Free Radical Scavengers / Q000494:pharmacology*; D006861:Hydrogen Peroxide / Q000032:analysis; D017665:Hydroxyl Radical / Q000032:analysis; D008550:Melatonin / Q000494:pharmacology*; D010100:Oxygen / Q000032:analysis; D017382:Reactive Oxygen Species; D026082:Singlet Oxygen; D013481:Superoxides / Q000032:analysis
chemical_list: D000975:Antioxidants; D003497:Cyclic N-Oxides; D016166:Free Radical Scavengers; D017382:Reactive Oxygen Species; D013481:Superoxides; D026082:Singlet Oxygen; D017665:Hydroxyl Radical; D006861:Hydrogen Peroxide; D008550:Melatonin; D010100:Oxygen; C003959:TEMPO