The role of sulphur, sulphide and reducible dyes in the enzymic oxidation of cysteamine to hypotaurine.

1. Cysteamine is oxidized to hypotaurine by an enzyme extracted from horse kidney, with sulphur or sulphide acting as a cofactor. It has been now found that, when the enzyme is omitted, sulphur and sulphide are able to catalyse the oxidation of cysteamine to cystamine by molecular oxygen. 2. Methylene blue may be used in catalytic amounts as a cofactor in the enzymic oxidation of cysteamine to hypotaurine in the place of sulphur or sulphide. The effect of methylene blue is not light-dependent and is not abolished by catalase. Other redox dyes with E'(0) higher than that of methylene blue are also used as cofactors. 3. A property common to all the cofactors is that they are necessary for the enzymic process in catalytic amounts, though they depress the final amount of hypotaurine produced when added over a critical concentration. All the cofactors share also the property of being catalysts for the non-enzymic oxidation of cysteamine to cystamine. 4. Methylene blue is reduced by cysteamine under anaerobic conditions, and is reoxidized in the presence of air. The rate of the reduction is not accelerated by the enzyme, indicating that the dye does not act in this reaction as a hydrogen carrier from the enzyme to oxygen. The possible mechanism of action of methylene blue and of the other cofactors is discussed.