CR(III)-MEDIATED HYDROXYL RADICAL GENERATION VIA HABER-WEISS CYCLE

Xanthine and xanthine oxidase were used as a source of superoxide (O-2 (.-)) and H2O2. A mixture of xanthine and xanthine oxidase in the pres ence of the spin trapping agent, 5,5-dimethyl-pyrroline N-oxide (DMPO) , generated DMPO/O-2(.-). Addition of Cr(III) to this mixture generate d DMPO/(OH)-O-.. Catalase partially inhibited DMPO/(OH)-O-'. formation , while the combination of catalase and superoxide dismutase (SOD) com pletely blocked the generation of DMPO/(OH)-O-.. The reaction of Cr(II I) with H2O2, itself, also generated DMPO/(OH)-O-.. This H2O2 enhanced DMPO/(OH)-O-. formation was significantly increased in the presence o f xanthine, and xanthine oxidase. Metal chelators, deferoxamine, 1,10- deferoxamine and EDTA, decreased Cr(III)-dependent (OH)-O-. generation . Parallel ESR spin trapping measurements were carried out using Cr(VI ), Although Cr(III) generated (OH)-O-. via a Haber-Weiss cycle, the re lative yield of the (OH)-O-. formation was comparable to that of a Fe( II)-mediated one but lower than that generated by a Cr(VI)mediated Hab er-Weiss cycle. HPLC measurements also show that the (OH)-O-. radical generated via a Cr(III)-mediated Haber-Weiss reaction was capable of c ausing 2'-deoxyguanosine (dG) hydroxylation to generate 8-hydroxyl-2'- deoxyguanosine (8-OHdG). The relative yield of 8-OHdG formation correl ated with the generation of (OH)-O-. as measured by ESR spin trapping. The results suggest that Cr(III)-mediated (OH)-O-. radical generation may contribute to the mechanism of Cr(III)- and Cr(VI)-induced carcin ogenesis. (C) 1998 Elsevier Science Inc. All rights reserved.