Synergism of ascorbic acid and glutathione in the reduction of hexavalent chromium in vitro.

The reduction of 20 microM hexavalent chromium [chromium(VI)] in single-reductant solutions of 0.02-2 mM L-ascorbic acid (L-AsA) or 2 mM reduced glutathione (GSH), and in mixed solutions of 2 mM GSH containing L-AsA of the above-described concentrations was investigated at physiological pH (37 degrees C). The reduction of chromium(VI) in the L-AsA solutions of higher concentrations showed pseudo-first-order kinetics in a single phase with respect to chromium(VI). In the lower L-AsA concentrations, pseudo-first-order processes appeared only in an early stage of the reaction. The reduction in the GSH solution was characterized by a two-phase process. In the first phase, which appeared in very short duration, the reaction rate was much higher than that in the second, but resulted in a decrease of only 7% of the initial chromium(VI) level. The second phase showed the main process of the reduction. This indicated pseudo-first-order kinetics. The half-life values of chromium(VI) depended on the initial levels of the reductants and ranged from seconds to hours. The chromium(VI)-reducing ability of L-AsA was markedly higher than that of GSH. In the mixed solutions of L-AsA and GSH, synergistic effect on the reduction of chromium(VI) was observed, especially in earlier stages of the processes. The synergism was more conspicuous in the mixed solutions containing lower L-AsA levels. These results suggest that although AsA possesses higher reducing ability than GSH, their coexistence is of importance for the reduction of chromium(VI) in biological systems because of their synergism.