The Standard Redox Potential of Cysteine-Cystine from the Thiol-Disulphide Exchange Reaction with Glutathione and Lipoic Acid

Thiol-disulphide exchange between glutathione (GSH) and cystine (CSSC) or oxidised glutathione (GSSG) and cysteine (CSH) has been followed by assaying GSH and GSSG specifically at low pH during the reaction. Velocity and equilibrium constants have been derived. From the latter and the known standard oxidation-reduction potential (E0′) of GSH/GSSG, E0′ (pH 7) of CSH/CSSC is found to be -0.22 volt. The course of the reaction of both these pairs with dihydrolipoic acid/lipoic acid has been inferred from absorbance changes at 330 mμ. Results confirm that their E0′ values are considerably more positive than for the dihydrolipoic acid/lipoic acid system.