Reduction of hexavalent chromium by ascorbic acid and glutathione with special reference to the rat lung

Abstract

The reduction of 20 μM hexavalent chromium [chromium(VI)] byl-ascorbic acid (AsA) (0.06–2 mM) and/or glutathione (GSH) (2–15 mM) in buffer solutions, cell-free bronchoalveolar lavage fluids or soluble fractions of rat lungs was investigated at physiological pH (37° C). The reduction in AsA solution was pseudo-first-order in a single phase with respect to chromium(VI), but that in GSH solution showed a two-phase process. The half-life of chromium(IV) ranged from seconds to hours. The reducing ability of AsA was markedly higher than that of GSH. Coexistence of equimolar GSH with AsA accelerated the reduction rate slightly, in comparison with that in the corresponding AsA solution. Lavage fluids containing 0.06 mM AsA showed pH-dependent reactions similar to those of the corresponding AsA solutions. The lungsoluble fractions reduced chromium(VI) in a process composed of phase I and phase II, characterized by the reducing ability of AsA-GSH cooperation and of AsA alone, respectively. Reduction in the former was 30–40% more rapid than in the latter. The biological half-life of chromium(VI) in the lung was estimated to be 0.6 min, on the basis of the reducing activity in the first phase. However, the apparent biological half-life of chromium(VI) was about 2 min in rat lungs after intratracheal injection of chromate, involving depletion of AsA, but no significant changes in GSH. The difference is discussed in terms of AsA-induced initiative reduction in the alveolar lining fluid and subsequent obstructive effects of the resulting trivalent species on trans-membrane permeability of chromate anions. These results suggest that AsA is more reactive than GSH in the reduction of chromium(VI) in the rat lung and that the extracellular AsA in the alveolar lining fluid plays an important role in antioxidant defense against inhaled chromium(VI) compounds.