To understand the possible mechanism of nitric oxide (NO) mediated cytotoxicity we investigated the effect of NO on endogenous antioxidant enzymes (AOEs) in rat C6 glial cells under conditions in which these cells expressed oligodendrocytelike properties. The 24h treatment with S-nitroso-N-acetylpenicillamine (SNAP), a NO donor, decreased the activities and the protein levels of catalase, glutathione peroxidase (GPX) and Mn-superoxide dismutase (SOD) in a dose dependent manner. On the other hand, the activity and the protein level of CuZnSOD were increased. A NO scavenger, PTIO, blocked the effect of SNAP. Moreover, the treatment of C6 cells with sodium nitroprusside, another NO donor, or with a combination of lipopolysaccharide (LPS) and interferon-γ (IFN-γ), which induce excessive production of NO also significantly modulated the AOE activities in a manner similar to that seen with SNAP treatment. Treatment with SNAP and a combination of LPS and IFN-γ also modulated the mRNA levels of AOEs parallel to the changes in their protein levels and activities except for MnSOD where the combination of LPS and IFN-γ markedly stimulated the mRNA expression. In spite of the stimulation of mRNA level, LPS and IFN-γ significantly inhibited the activity of MnSOD within the first 24 h of incubation, however, MnSOD activity gradually increased with the increase in time of incubation. These results suggest that alterations in the status of AOEs by NO may be the basis of NO induced cytotoxicity in disease states associated with excessive NO production.(Funded by a grant NS-22576 from N.I.H.)