The reduction potential of cysteine

IT has been shown by Dixon and Quastel [1923] that the reduction potential of cysteine-cystine mixtures depends entirely on the concentration of the reduced form, cysteine. Cystine seems to exert no potential under the experi- mental conditions employed. The suggestion made to explain this anomalous behaviour was contained in the following words: " It seems almost necessary, therefore, to postulate the possible presence of an intermediate compound which would be the " true " oxidised form of cysteine and which is transformed irreversibly into cystine. In such a case the concentration of this intermediate oxidised substance would appear to be maintained at a constant value, as though the solution in presence of cysteine must be " saturated " with respect to this particular compound." Dixon [1927] has advanced another interpretation of this phenomenon based upon an extension of the Wieland theory of activation of hydrogen. According to Dixon, when an electrode is placed in a solution containing a substance AH2 which gives a reduction potential, the electrode surface becomes