Inhibition of Plasma Monoamine Oxidase by Cysteamine

CYSTEAMINE has been shown to cause a transient inhibition of diamine oxidase1, which was explained by its coupling with pyridoxal phosphate in a thiazolidine ring. Even when linked to pyridoxal phosphate thiols may undergo oxidation to the corresponding disulphides2,3 in the case of cysteamine the non-enzymatic oxidation to cystamine liberates pyridoxal phosphate, and the diamine oxidase activity is restored. Because the enzyme oxidizes cystamine4, we have an interesting example of an inhibitor which is changed into a substrate during the reaction.