Cr(III)-mediated hydroxyl radical generation via Haber-Weiss cycle

ESR spin trapping and HPLC were utilized to investigate Cr(III)-mediated hydroxyl radical (·OH) generation via the following Haber-Weiss reactions in vitro: Cr(III) + O₂/(·-) → Cr(II) + O₂ Cr(II) + H₂O₂ → Cr(III) + ·OH + OH^- Xanthine and xanthine oxidase were used as a source of superoxide (O₂/(·-)) and H₂O₂. A mixture of xanthine and xanthine oxidase in the presence of the spin trapping agent, 5,5-dimethyl-pyrroline N-oxide (DMPO), generated DMPO/O₂/(·-). Addition of Cr(III) to this mixture generated DMPO/·OH. Catalase partially inhibited DMPO/·OH formation, while the combination of catalase and superoxide dismutase (SOD) completely blocked the generation of DMPO/·OH. The reaction of Cr(III) with H₂O₂, itself, also generated DMPO/·OH. This H₂O₂ enhanced DMPO/·OH formation was significantly increased in the presence of xanthine, and xanthine oxidase. Metal chelators, deferoxamine, 1,10-deferoxamine and EDTA, decreased Cr(III)- dependent ·OH generation. Parallel ESR spin trapping measurements were carried out using Cr(VI). Although Cr(III) generated ·OH via a Haber-Weiss cycle, the relative yield of the ·OH formation was comparable to that of a Fe(II)-mediated one but lower than that generated by a Cr(VI)-mediated Haber- Weiss cycle HPLC measurements also show that the ·OH radical generated via a Cr(III)-mediated Haber-Weiss reaction was capable of causing 2'- deoxyguanosine (dG) hydroxylation to generate 8-hydroxyl-2'-deoxyguanosine (8-OHdG). The relative yield of 8-OHdG formation correlated with the generation of ·OH as measured by ESR spin trapping. The results suggest that Cr(III)-mediated ·OH radical generation may contribute to the mechanism of Cr(III)- and Cr(VI)-induced carcinogenesis.