Different inhibition characteristics of intracellular transglutaminase activity by cystamine and cysteamine

The treatment of cystamine, a transglutaminase(TGase) inhibitor, has beneficial effects in several diseases including CAG-expansion disorders and cataract. We compared the inhibition characteristics of cystamine with those of cysteamine, a reduced form of cystamine expect-ed to be present inside cells. Cystamine is a more potent inhibitor for TGase than cysteamine with different kinetics pattern in a non-reducing condition. By contrast, under reducing conditions, the inhibitory effect of cystamine was comparable with that of cysteamine. How-ever, cystamine inhibited intracellular TGase activity more strongly than cysteamine despite of cytoplasmic reducing environment, suggest-ing that cystamine itself inhibits in situ TGase activity by forming mixed disulfides.