Activation of Molecular Oxygen during the Reactions of Chromium(VI/V/IV) with Biological Reductants: Implications for Chromium-Induced Genotoxicities1

Abstract

The first systematic study on the roles of O2 in the reactions of Cr(VI/V/IV) with major intracellular reductants, cysteine (1), glutathione (2), and ascorbic acid (3) as well as with vitamin E analogue Trolox (4), has been performed. The reactions of 1−3 with Cr(VI) (aqueous buffer solutions, pH = 4.5−7.5, 25 °C) led to a slow O2 consumption (measured by a Clark oxygen electrode). The reactions of 1−3 with the relatively stable Cr(V) and Cr(IV) 2-ethyl-2-hydroxybutanoato complexes under the same conditions were accompanied by fast O2 consumption. The O2 consumption during the reactions of Cr(VI/V/IV) with 1−3 did not lead to a significant accumulation of H2O2 (determined with catalase). No significant O2 consumption was detected for the reactions of Cr(VI/V/IV) with 4. To reveal the mechanisms of O2 activation, the kinetics of the Cr(V/IV) reactions with 1−4 at pH 4.5 and 7.5 were studied by stopped-flow UV−visible spectrophotometry; and the kinetic data were processed by the global analysis method. The stoichiometries and products of these reactions were studied by UV−visible, CD, and EPR spectroscopies. The proposed mechanism of O2 activation includes oxidations of 1−4 by Cr(V/IV) to produce organic radical intermediates, which then react with O2 in chain processes. No evidence was found for the direct activation of O2 by the Cr compounds. Implications of the proposed mechanism to the DNA damage induced by the Cr(VI) + reductant + O2 systems have been discussed.