NITRIC-OXIDE PROTECTS AGAINST CELLULAR-DAMAGE AND CYTOTOXICITY FROM REACTIVE OXYGEN SPECIES

Nitric oxide, NO, which is generated by various components of the immu ne system, has been presumed to be cytotoxic. However, NO has been pro posed to be protective against cellular damage resulting during ischem ia reperfusion. Along with NO there is often concomitant formation of superoxide/hydrogen peroxide, and hence a synergistic relationship bet ween the cytotoxic effects of nitric oxide and these active oxygen spe cies is frequently assumed. To study more carefully the potential syne rgy between NO and active oxygen species in mammalian cell cytotoxicit y, we utilized either hypoxanthine/xanthine oxidase (a system that gen erates superoxide/hydrogen peroxide) or hydrogen peroxide itself. NO g eneration was accomplished by the use of a class of compounds known as ''NONOates,'' which release NO at ambient temperatures without the re quirement of enzyme activation or biotransformation. When Chinese hams ter lung fibroblasts (V79 cells) were exposed to hypoxanthine/xanthine oxidase for various times or increasing amounts of hydrogen peroxide, there was a dose-dependent decrease in survival of V79 cells as measu red by clonogenic assays. However, in the presence of NO released from (C2H5)2N[N(O)NO]-Na+ (DEA/NO), the cytotoxicity resulting from supero xide or hydrogen peroxide was markedly abrogated. Similarly, primary c ultures of rat mesencephalic dopaminergic cells exposed either to hydr ogen peroxide or to hypoxanthine/xanthine oxidase resulted in the degr adation of the dopamine uptake and release mechanism. As was observed in the case of the V79 cells, the presence of NO essentially abrogated this peroxide-mediated cytotoxic effect on mesencephalic cells.