Oxidation of melatonin by carbonate radicals and chemiluminescence emitted during pyrrole ring cleavage


Address reprint requests to Rüdiger Hardeland, Institute of Zoology and Anthropology, University of Göttingen, Berliner Str. 28, D-37073 Göttingen, Germany. E-mail: rhardel@gwdg.de


Abstract: Oxidation of melatonin was followed by measuring chemiluminescence emitted during pyrrole ring cleavage, a process leading to the main oxidation product of this indoleamine, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK). Radical reactions of melatonin were studied in two variants of a moderately alkaline (pH 8) H2O2 system, one of which contained hemin as a catalyst. In both systems, light emission from melatonin oxidation lasted several hours. Time courses and turnover rates depended on the presence or absence of hemin; the catalyst enhanced light emission many-fold. In the two reaction systems, the presence of hydrogen carbonate (HCOinline image) enhanced chemiluminescence by more than 10-fold, indicating scavenging of carbonate radicals. In the presence of 10% dimethylsulfoxide (DMSO) or 1 m mannitol, HCOinline image-dependent as well as independent light emissions were only partially inhibited. With regard to the stimulatory effect of HCOinline image, this implies a formation of carbonate radicals (inline image) independent of hydroxyl (OH) radicals, presumably involving superoxide anions abundantly present in the system. Tiron, a scavenger of superoxide anions, strongly and almost instantaneously inhibited chemiluminescence, in accordance to the requirement of this reactive oxygen species for AFMK formation and its involvement in inline image-radical formation. Melatonin's capability of scavenging COinline image may contribute to its protective potency.