The reduction of chromium (VI) to chromium (III) by glutathione: an intracellular redox pathway in the metabolism of the carcinogen chromate

Abstract

The capacity of glutathione (GSH) to reduce Cr(VI) to Cr(III) in vitro was investigated. The reaction was determined spectrophotometrically by following the absorption of Cr(VI) at 370 nm. At stoichiometric conditions (molar ratio Cr(VI)/GSH of 1:3) the reduction was strongly dependent on the solution's pH. It was much slower at pH 7.4 than at pH values below 5. An excess of GSH (100- or 1000-fold) accelerated the reaction. In any case, 3 GSH molecules were required to reduce 1 molecule of chromate. Incubation of human red blood cells (RBC) with an excess of Na2CrO4 (10 mM) decreased the GSH content of the cells to 10% of the original amount. This depletion of GSH was similar to that obtained when RBC were incubated with 62 mM diethylmaleate (DEM), a well known GSH depleting agent. Sephadex G-100 chromatography of lysates from human RBC incubated with radioactive chromate (51Cr(VI] showed a strong affinity of 51Cr for hemoglobin: 97% of the applied dose was bound to hemoglobin whilst only minor amounts of 51Cr were found in the low-molecular fractions. However, incubations of prepared lysates (as opposed to intact cells) with 10 mM Na2 51CrO4 markedly raised the chromium content of low-molecular fractions (probably GSH-Cr-complexes), probably indicative of a role of GSH in the intra-cellular reduction of Cr(VI) to Cr(III), the latter being regarded as the ultimately toxic species of this metal.