Chromium (VI) reducing capacity of ascorbic acid and of human plasma in vitro

In the metabolism of chromium(VI) its reduction in human plasma is of importance; an extracellular reduction of Cr(VI) is regarded as a detoxification step. Ascorbic acid has been suggested to represent the majority of the Cr(VI)-reducing capacity of human plasma. Therefore the kinetics of the reaction of Cr(VI) with ascorbic acid, at biologically realistic concentrations were studied. Ascorbic acid, in 0.2 M HEPES buffer and at concentrations ranging from 14.2 to 113.6 nmol ml-1 (2.5-20.0 microgram ml-1), was mixed with Cr(VI) (0.4-1.5 nmol ml-1) and incubated at pH 7.4 and 37 degrees C. In addition, chromate solutions at different concentrations [1.5-100 nmol ml-1 Cr(VI)], were incubated at 37 degrees C with freshly drawn blood. From these incubates, ascorbic acid and its oxidized form, dehydroascorbic acid, were simultaneously analyzed by HPLC and post-column derivatization. Chromate was determined by flow injection analysis. The reaction kinetics of ascorbic acid in HEPES buffer with Cr(VI) is of pseudo-first order at higher concentrations, whilst apparently at lower concentrations kinetics are consistent with an autocatalyzed reaction. Results obtained after spiking human plasma are similar. However, when Cr(VI) was reacted with human plasma, no changes in the intrinsic contents of ascorbic acid of the plasma samples occurred. Also, comparing different plasma samples the intrinsic plasma contents of ascorbic acid and the reduction capacities for Cr(VI) [ranging between 0.48 and 0.63 nmol ml-1 Cr(VI) to be reduced] did not correlate. This shows that the reduction of Cr(VI) in native human plasma is complex and is not only determined by the plasma ascorbic acid levels. This is in contrast to the situation in lung lavage fluids (Suzuki 1988; Suzuki and Fukuda 1990) where the concentrations of ascorbic acid are much higher than in blood.