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

Authors


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

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.

Ancillary