The mechanism of cytotoxicity of the NO donor 3-morpholino-sydnonimine toward a human ovarian cancer cell line (OVCAR) was examined. It was found that the NO-mediated loss of cell viability was dependent on both NO and hydrogen peroxide (H 2 O 2 ). Somewhat surprisingly, superoxide (O 2 . ) and its reaction product with NO, peroxynitrite ( ؊ OONO), did not appear to be directly involved in the observed NO-mediated cytotoxicity against this cancer cell line. The toxicity of NO/H 2 O 2 may be due

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... to the production of a potent oxidant formed via a trace metal-, H 2 O 2 -, and NO-dependent process. Because the combination of NO and H 2 O 2 was found to be particularly cytotoxic, the effect of NO on cellular defense mechanisms involving H 2 O 2 degradation was investigated. It was found that NO was able to inhibit catalase activity but had no effect on the activity of the glutathione peroxidase (GSHPx)-glutathione reductase system. It might therefore be expected that cells that utilize primarily the GSHPx-glutathione reductase system for degrading H 2 O 2 would be somewhat resistant to the cytotoxic effects of NO. Consistent with this idea, it was found that ebselen, a compound with GSHPx-like activity, was able to protect cells against NO toxicity. Also, lowering endogenous GSHPx activity via selenium depletion resulted in an increased susceptibility of the target cells to NO-mediated toxicity. Thus, a possible NO/H 2 O 2 /metal-mediated mechanism for cellular toxicity is presented as well as a possible explanation for cell resistance/susceptibility to this NO-initiated process.