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We examined the protective effects of N-acetylcysteine (NAC) on the death of glia-free neurons in culture. Under normoxic conditions, the protection by NAC was observed only in cystine-free but not complete medium. When the cells were cultured under hypoxic conditions, NAC much elongated their survival even in the presence of cystine. H2O2 was found to be generated to considerable concentration in the presence of both NAC and cystine, and the administration of catalase prevented the cell death. These results suggest that the harmful effect of NAC is because of H2O2 generated by autoxidation of cysteine, which derives from the reaction between NAC and cystine. The present results raise the possibility that NAC can act as either antioxidant or prooxidant depending on the milieu.