Cellular antioxidant and pro-oxidant actions of nitric oxide

We describe a biphasic action of nitric oxide (NO) in its effects on oxidat ive killing of isolated cells: low concentrations protect against oxidative killing, while higher doses enhance killing, and these two effects occur b y distinct mechanisms. While low doses of NO (from (Z)-1-[N-(3-ammonio prop yl)-N-(n-propyl)-amino]-diazen-1-ium-1,2(2) diolate [PAPA/NO] or S-nitroso- N-acetyl-L-penicillamine [SNAP] prevent killing of rat hepatocytes by t-but ylhydroperoxide (tBH), further increasing doses result in increased killing . Similar effects occur with rat hepatoma cells treated with PAPA/NO and tB H or H2O2. Increased killing with higher concentrations of NO donor is due to both NO and tBH, because NO donor alone is without effect. Glutathione ( GSH) is not involved in either of these actions. Based on measurements of t hiobarbituric acid-reactive substances (TBARS) and effects of lipid radical scavenger (DPPD) and deferoxamine, the protective effect, but not the enha ncing effect, involves peroxidative chemistry. Fructose has no effect on tB H killing alone but provides substantial protection against killing by high er concentrations of NO plus tBH, suggesting that the enhancing effect invo lves mitochondrial dysfunction. Hepatocytes, when stimulated to produce NO endogenously, become resistant to tBH killing, indicative of the presence o f an NO-triggered antioxidant defensive mechanism. The finding that the pro tective effects of low concentrations of NO and the harmful effects of high concentrations of NO are fundamentally different in nature suggest that th erapeutic interventions could be designed, which selectively prevent its pr o-oxidant activity at high concentrations, thus converting NO from a "Janus -faced" modulator of oxidant injury into a "pure" protectant. (C) 1999 Else vier Science Inc.