THE CHEMISTRY AND TUMORICIDAL ACTIVITY OF NITRIC-OXIDE HYDROGEN-PEROXIDE AND THE IMPLICATIONS TO CELL RESISTANCE SUSCEPTIBILITY

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 bot h NO and hydrogen peroxide (H2O2). Somewhat surprisingly, superoxide ( O-2(-.)) and its reaction product with NO, peroxynitrite ((OONO)-O--), did not appear to be directly involved in the observed NO-mediated cy totoxicity against this cancer cell line. The toxicity of NO/H2O2 may be due to the production of a potent oxidant formed via a trace metal- , H2O2-, and NO-dependent process. Because the combination of NO and H 2O2 was found to be particularly cytotoxic, the effect of NO on cellul ar defense mechanisms involving H2O2 degradation was investigated. It was found that NO was able to inhibit catalase activity but had no eff ect on the activity of the glutathione peroxidase (GSHPx)-glutathione reductase system. It might therefore be expected that cells that utili ze primarily the GSHPx-glutathione reductase system for degrading H2O2 would be somewhat resistant to the cytotoxic effects of NO. Consisten t with this idea, it was found that ebselen, a compound with GSHPx-lik e activity, was able to protect cells against NO toxicity. Also, lower ing endogenous GSHPx activity via selenium depletion resulted in an in creased susceptibility of the target cells to NO-mediated toxicity. Th us, a possible NO/H2O2/metal-mediated mechanism for cellular toxicity is presented as well as a possible explanation for cell resistance/sus ceptibility to this NO-initiated process.