Autoxidation of cysteine generates hydrogen peroxide: cytotoxicity and attenuation by pyruvate.

The reactivity of cysteine presents a paradox: although regarded as an antioxidant, cysteine interacts with oxygen in a metal-catalyzed reaction to produce reactive species. Because ischemia provokes the appearance of millimolar amounts of cysteine and increased amounts of transition metals, we studied whether cysteine, in the presence of transition metals, consumes oxygen, generates hydrogen peroxide, and is toxic. Using fluorescence cytometry, we provide direct evidence that hydrogen peroxide is copiously generated during cysteine autoxidation. Pyruvate attenuates such generation of hydrogen peroxide and cytotoxicity. Cysteine oxidation is stimulated by an EDTA-chelatable diethyl-dithiocarbamate-chelatable constituent of kidney extract; this suggests that copper is the catalytically active metal. The toxicity resulting from cysteine oxidation is less than that induced by amounts of reagent hydrogen peroxide that produce comparable fluorescence. Cysteine also prevents hydrogen peroxide-induced toxicity. Thus, although cysteine generates hydrogen peroxide, it can guard against hydrogen peroxide toxicity, possibly by binding metals on which the toxicity of hydrogen peroxide is dependent. Thus the behavior of cysteine can be salutary or pernicious; the net effect of cysteine, within this wide ambit of actions, is decisively influenced by the conditions to which cysteine is exposed.