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

Abstract

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.

Use of molecular epidemiological techniques in a pilot study on workers exposed to chromium.

“…'2 A recent study reported that plasma reducing capabilities for chromium(VI) ranged from 25 to 33 #g/i, however, these reducing capabilities did not correlate with plasma ascorbic acid concentration. 34 There was no significant effect of chromium exposure on damage to DNA, measured as DNA strand breaks and formation of 8-OHdG, in the present study. This accords with the findings of an animal study in which a single intratracheal dose of sodium dichromate (004 mg Cr/kg), approximately corresponding to an exposure level at the MEL of 005 mg/M3 over an 8 h working day, was not found to induce increased DNA strand breaks in rat lymphocytes compared with controls.20 This is also consistent with the results of chromium concentration in lymphocytes.…”

Structural and magnetic studies on mono- and polynuclear chromium ascorbate complexes

“…Under physiological conditions, Cr VI is reduced by microsome, mitochondria and by some non-enzymatic cellular components. As a cellular component, ascorbic acid is known to reduce Cr VI to Cr III in vivo [1,2] and through inner-sphere electron transfer reactions in vitro [3,4]. The final Cr III ascorbate complexes may be of considerable interest in biocoordination chemistry aiding a better understanding of chromium metabolism and also targeting miscellaneous potential applications.…”

The role of glutathione in the acute nephrotoxicity of sodium dichromate

“…On this basis, the extracellular Cr(VI) reduction is considered to represent a detoxification mechanism (Jones 1990;Suzuki and Fukuda 1990;Capellmann and Bolt 1992). Cr(VI) is subject to subsequent reduction to Cr(III) within the cell (Connett and Wetterhahn 1983).…”