Kinetic Studies on Decomposition of Glutathione. III. Peptide Bond Cleavage and Desulfurization in Aqueous Solution

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Kinetic studies on the peptide bond (γ-glutamyl bond) cleavage and desulfurization of glutathione (GSH) in anaerobic aqueous solution were performed in the range of pH 1-12 at 60°. The desulfurization of GSH in neutral solution was very slow, but the rate increased markedly with increase of pH. In the range of pH 10-12, the pH of the reaction solution had little effect on the desulfurization rate. The cleavage rate of γ-glutamyl bond showed a minimum in the range of pH 5-6, a maximum at pH 8.5 and a plateau in the range of pH 10-12. The effects of ionic strength and dielectric constant on both γ-glutamyl bond cleavage and desulfurization were almost negligible. The apparent activation energy for cleavage of the γ-glutamyl bond was 19-21 kcal/mol and that for desulfurization was about 21 kcal/mol. The apparent first-order rate constants of both reactions were related to the mole fractions of ionic species of GSH and could be expressed as a function of hydrogen ion activity at arbitrary pH. In the range of pH 6.5-10, the pH profile of the logarithmic cleavage rate constant of the γ-glutamyl bond was bell-shaped ; it is likely that cleavage of the γ-glutamyl bond is accelerated, at least in the range of pH 7.5-10, by intramolecular catalysis involving the NH₂ and SH groups of GSH.