Chromate-mediated free radical generation from cysteine, penicillamine, hydrogen peroxide, and lipid hydroperoxides.

The Cr(VI)-mediated free radical generation from cysteine, penicillamine, hydrogen peroxide, and model lipid hydroperoxides was investigated utilizing the electron spin resonance (ESR) spin trapping technique. Incubation of Cr(VI) with cysteine (Cys) generated cysteinyl radical. Radical yield depended on the relative concentrations of Cr(VI) and Cys. The radical generation became detectable at a cysteine:Cr(VI) ratio of about 5, reached its highest level at a ratio of 30, and declined thereafter. Cr(VI) or Cys alone did not generate a detectable amount of free radicals. Similar results were obtained with penicillamine. Incubation of Cr(VI), Cys or penicillamine and H2O2 led to hydroxyl (.OH) radical generation, which was verified by quantitative competition experiments utilizing ethanol. The mechanism for .OH radical generation is considered to be a Cr(VI)-mediated Fenton-like reaction. When model lipid hydroperoxides such as t-butyl hydroperoxide and cumene hydroperoxide were used in place of H2O2, hydroperoxide-derived free radicals were produced. Since thiols, such as Cys, exist in cellular systems at relatively high concentrations, Cr(VI)-mediated free radical generation in the presence of thiols may participate in the mechanisms of Cr(VI)-induced toxicity and carcinogenesis.