Production and utilization of hydrogen peroxide associated with melanogenesis and tyrosinase-mediated oxidations of DOPA and dopamine

The synthesis and involvement of H2O2 during the early stages of melanogenesis involving the oxidations of DOPA and dopamine (diphenolase activity) were established by two sensitive and specific electrochemical detection systems. Catalase-treated reaction mixtures showed diminished rates of H2O2 production during the autoxidation and tyrosinase-mediated oxidation of both diphenols. Inhibition studies with the radical scavenger resveratrol revealed the involvement in these reactions of additional reactive intermediate of oxygen (ROI), one of which appears to be superoxide anion. There was no evidence to suggest that H2O2 or any other ROI was produced during the tyrosinase-mediated conversion of tyrosine to DOPA (monophenolase activity). Establishing by electrochemical methods the endogenous production H2O2 in real time confirms recent reports, based in large part on the use of exogenous H2O2, that tyrosinase can manifest both catalase and peroxidase activities. The detection of ROI in tyrosinase-mediated in vitro reactions provides evidence for sequential univalent reductions of O-2, most likely occurring at the enzyme active site copper. Collectively, these observations focus attention on the possible involvement of peroxidase-H2O2 systems and related ROI-mediated reactions in promoting melanocytotoxic and melanoprotective processes.