The determination of kinetic data for the reactions of chromium(VI) with glutathione and other thiols

Abstract

The equations necessary for the determination of the rate constants corresponding to the opposite elementary steps for formation of the oxidant-reductant ester intermediate, as well as that corresponding to its redox decomposition, in the reduction of chromiumWI) by glutathione and other thiols at physiological pH have been deduced. It is shown that those three rate constants can be obtained with a good reproducibility. 0 1994 John Wiley & Sons, Inc.In recent years, there has been a growing interest in elucidating the mechanism through which chromium(VI), one of the most potent inorganic carcinogens [ll, exerts its carcinogenic action [2,31. The reaction between chromium(V1) and glutathione seems to be of some relevance to that respect, since high cellular levels of the latter substance are known to increase the number of chromium-induced DNA mutations [4].It seems clear from previous reports [5-73 that the opening steps of the mechanism for the reduction of chromium(VI) by glutathione (GSH) are the formation of an oxidant-reductant ester intermediate (GSCrvI) and the redox decomposition of the latter by reaction with a second thiol molecule to give chromium(1V) and the main organic reaction product (glutathionyl disulfide, GSSG).Although the kinetics of the reaction in perchloric acid media has received some attention [51, the studies at physiological pH are concerned mainly with the identification of some intermediates (chromium(V) [8-101, glutathionyl radical [lo], and the oxidant-reductant ester intermediate [61) rather than with the determination of rate constants. Given that it is generally accepted that the predominant reaction pathway involves a two-electron reduction of chromium(VI) [eq. (2)1, the appearance of GS. as intermediate is probably due to a one-electron pathway that, in spite of being secondary (at least, under conditions of a large excess of GSH), might have some relevance as far as the biological implications of the reaction are concerned [lll.

Formation of Superoxochromium(III) by a Novel Mechanism

“…In the same reaction, performed in aerated aqueous solutions at neutral pH, an acceleration by manganese(II) has been observed [34], a significant result in view of the fact that manganese(II) inhibits the reactions of chromium(VI) with most organic reductants in acid media, due to its ability to act as an efficient trapping agent for the intermediate chromium(IV), thus preventing its 2-electron reduction to chromium(II) and its posterior reaction with the reactant chromium(VI) [Equation (14)] [46][47][48]. The catalytic effect of manganese(II) in the present case is consistent with the decrease observed in the yield of CrOO 2+ after its addition (Figure 8), thus preventing the reaction of this intermediate with O 2 to recuperate the reactant chromium(VI) [Equations (19) and (20)]. According to this explanation, the role of Mn II would be that of suppressing the inhibition of the chromate + L-cysteine reaction caused by O 2 (Figure 1).…”

“…In both cases the decomposition of CrOO 2+ produced CrO 2À 4 , as evidenced by its absorption at 370 nm, but the yield clearly increased as the oxygen concentration increased, in agreement with Equations (19) and (20). In the case of the argon-saturated solution (½O 2 =½CrOO 2þ ¼ 9), 72% of the initial CrOO 2+ was recuperated as CrO 2À 4 , whereas in the case of the air-saturated solution (½O 2 =½CrOO 2þ ¼ 39) the corresponding yield was 84%.…”

“…Although the former reaction must be taken only as a hypothesis to explain the experimental results presented in this work, several findings consistent with Equations (19) and (20) (20)], which decays in a second step to yield CrOO 2+ [reverse reaction of Equation (19)] [18]. During the chromate + L-cysteine reaction in neutral aqueous solutions a decay in the concentration of O 2 and an accumulation of H 2 O 2 have been observed [57].…”

“…In both cases the decrease of the rate as the reaction advanced was faster than expected for a reaction first order in chromate (L-cysteine was in large excess), the deviation from pseudo-first-order kinetics being more pronounced in the air-saturated reaction. This can be interpreted in terms of Scheme 1 [19][20][21][22], where I is the long-lived chromium(VI) thioester intermediate HRSCrO À 3 (HRSH being L-cysteine) [23][24][25], whose formation and decay can be followed at 430 nm (typical lifetime % 10 min). The rates of formation of this intermediate in the argon-saturated and air-saturated reactions were very similar, but its decay was much faster in the argon-saturated reaction (Figure 1, top).…”

“…In an attempt to confirm that Equations (19) and (20) were correct, authenticated CrOO 2+ was prepared by reduction of chromium(VI) with 2-propanol in an air-saturated acid solution [7], and then injected into either an argon-saturated or an air-saturated Na 2 HPO 4 solution to reach near-neutrality conditions ( Figure 12). In both cases the decomposition of CrOO 2+ produced CrO 2À 4 , as evidenced by its absorption at 370 nm, but the yield clearly increased as the oxygen concentration increased, in agreement with Equations (19) and (20).…”