Reversible Oxidation - Reduction Systems of Cysteine - Cystine and Reduced

Investigation of the oxidation-reduction potentials of 2-oxy2,3-dihydroindole-3-propionic acid established the fact that the compound in both its reduced and oxidized forms has little effect on the platinum electrode (9). It has the ability, however, to react with dibromoindophenol, with hydrogen dioxide, molecular oxygen, and many other oxidizing agents. Interaction with these compounds produces within the solution variations in the reducing potential. When dibromoindophenol is used, the dye is completely reduced and a reducing potential of f0.1 volts can be produced. The oxidation of this lactam was shown to be of a simple stoichiometric nature, and the velocity curve indicated that an addition product was formed between the lactam and the oxidizing compound, probably through the amine group. It was suggested that this intermediate product is unstable, tending to break down with the removal of two atoms of hydrogen from the lactam. The reduced and oxidized forms of 2-oxydihydroindole-3propionic acid are not in equilibrium with each other, and they do not form a reversible oxidation-reduction system. No compound has been found which can bring the two forms into equilibrium. In 1923, Dixon and Quastel published an investigation of the oxidation-reduction potentials of cysteine and cystine and of reduced and oxidized glutathione (4). They suggested that these