Role of molecular oxygen in the generation of hydroxyl and superoxide anion radicals during enzymatic Cr(VI) reduction and its implication to Cr(VI)-induced carcinogenesis.
Leonard-SS; Wang-S; Zang-L; Castranova-V; Vallyathan-V; Shi-X
J Environ Pathol Toxicol Oncol 2000 Jan; 19(1-2):49-60
Electron spin resonance (ESR) spin trapping measurements provide evidence for the generation of hydroxyl radicals (OH) in the reduction of Cr(VI) by glutathione reductase (GSSG-R) in the presence of NADPH as a cofactor. Catalase inhibited the OH generation, while the addition of H2O2 enhanced it, indicating that the OH radical generation involves a Fenton-like reaction. The metal chelator, deferoxamine, inhibited the OH generation with a concomitant generation of a deferoxamine nitroxide radical. EDTA and 1,10-phenanthroline also inhibited the OH generation. Experiments performed under argon atmosphere decreased the yield of the OH formation, showing that molecular oxygen plays a critical role. ESR spin trapping and measurements of fluorescence change of scopoletin in the presence of horseradish peroxidase show that reduction of Cr(VI) by GSSG-R/NADPH generates superoxide anion radicals (O2-) as well as H2O2. It can be concluded that OH radical is generated by the reaction of H2O2 with Cr(V), which is produced by enzymatic one-electron reduction of Cr(VI). H2O2 is produced by the reduction of molecular oxygen via OH- as an intermediate. The OH radicals generated by these reactions are capable of causing DNA strand breaks, which can be inhibited by catalase, formate, and experiments performed under argon.
Carcinogenesis; Carcinogenicity; Carcinogens; Hydroxyl-groups; Chromium-compounds; Metals; Chelates; Chelating-agents
Xianglin Shi, Ph.D, Pathology and Physiology Research Branch, Health Effects Laboratory Division, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, 1095 Willowdale Road, Morgantown, WV 26505-2845
Journal of Environmental Pathology, Toxicology, and Oncology