The use of an exact integrated rate law has allowed the determination of the three rate constants associated with the formation (kl), decomposition (k-I), and reactivity (k2) of the thioester involved as a relativelystable intermediate in the reduction of chromium(V1) by glutathione in neutral or slightly acidic aqueous solution (pH = 4.0-7.2). The rate constants kl and k2 increase linearly as the concentration of glutathione increases, whereas k-1 does not change. Both kl and k-1 increase with rising concentration of buffer. The effect of three different buffering agents has been studied, the catalytic power of each buffer increasing in the order acetate < citrate < phosphate. At constant buffer concentration, the kl vs pH plots are bell-shaped, with a maximum at pH = 5-6. The three rate constants increase with rising ionic strength, and in the presence of acetate buffer, Zn2+ exerts a catalytic effect on both kl and k-1. The apparent activation energies associated with the rate constants kl, k-1, and k2 are 42.3 f 0.8, 61.6 f 1.4, and 35.7 f 3.3 kT mol-', respectively. A mechanism explaining the catalytic effects of both the buffer and Zn2+ and involving Cr(I1) as an unstable intermediate is proposed for the reaction. It is also suggested that either Cr(I1) or CxG2+ (formed from the reaction of the latter intermediate with oxygen) might be one of the mutagenic agents implicated in the carcinogenicity of chromium(V1) activated by glutathione.

, 1637 (1994). The kinetics of formation of the thioester involved as an intermediate in the reaction between chromium(V1) and or.-penicillamine in aqueous media (pH = 1-8) containing different buffers (acetate, citrate, and phosphate) has been studied by monitoring the disappearance of chromium(V1) at 370 nm and application of the initial-rates method. The initial rate is directly proportional to the initial concentrations of both oxidant and reductant, and the rate vs. pH plots show bell-shaped profiles. The reaction is catalyzed by the buffer present in the medium, the catalytic power of each buffer increasing in the order acetate < citrate < phosphate. This is explained in terms of a mechanism involving the formation of a complex between the acidic form of the buffer and HCr04-previous to the formation of the thioester. Potassium chloride and sodium sulfate do not seem to have important specific effects on the reaction rate, their effect being that of an acceleration of the reaction as the ionic strength increases. On the contrary, the sulfates of magnesium, manganese(II), and zinc (the latter only in the presence of acetate buffer) have specific effects, indicating the probable formation of several complexes. The spectrophotometric detection of the thioester at 430 nm has allowed to confirm some of the conclusions extracted from the measurement of initial rates, and suggests that this intermediate might approach a steady-state behavior in the three buffers at pH > 6.25, and also that a bimolecular reaction with DL-penicillamine might be involved in its destruction. JOAQUIN F. PEREZ-BENITO, DRISS LAMRHARI et CONCHITA ARIAS. Can. J. Chem. 72, 1637 (1994).En examinant la vitesse de disparition du chrome(V1) B 370 nm et en appliquant la mCthode des vitesses initiales, on a CtudiC la cinitique de formation du thioester impliquC comme intermkdiaire dans la rCaction du chrome(V1) et la DLpinicillamine, en milieu aqueux (pH = 1 B 8) contenant divers tampons (acCtate, citrate et phosphate). La vitesse initiale est directement proportionnelle aux concentrations B la fois de I'oxydant et du rCducteur et les courbes des vitesses en fonction du pH prksentent des profils en forme de cloche. La rCaction est catalysCe par le tampon prCsent dans le milieu, le pouvoir catalytique de chacun des tampons augmentant dans l'ordre acCtate < citrate < phosphate. On peut expliquer ces rCsultats en fonction d'un mCcanisme impliquant la formation, avant la formation du thioester, d'un complexe entre la forme acide du tampon et le HCr04-. Le chlorure de potassium et le sulfate de sodium ne semblent pas avoir d'effets spCcifiques importants sur la vitesse de la reaction; leur effet se limite B une accClCration de la rCaction lorsque la force ionique augmente. Au contraire, les sulfates de magnCsium, de manganbse(I1) et de zinc (dans ce dernier cas, uniquement en prCsence du tampon acCtate) posskdent des effets spCcifiques, suggCrant la formation probable de plusieurs complexes. La dCtection spectrophotomCtrique du thioester 2 430 nm a p...

The reaction between chromium(vi) and formaldehyde in aqueous perchloric acid is inhibited by manganese(i1) ion, according to the relation k, = a/(l + b[Mn2+]) + c; manganese(i1) ion and formaldehyde compete to reduce the intermediate Crlv, and Cr" is an intermediate.According to Westheimer's mechanisms,lJ there are in principle two possibilities for the reduction of CrVI to CrI" by two-equivalent reducing agents (R), one involving only CrV and CrlV as intermqdiates [eqn. (l)], and the other involving also Cr" as intermediate [eqn. (2)].

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.