Inhibitory and enhancing effects of NO on H2O2 toxicity: Dependence on the concentrations of NO and H2O2

Abstract

Nitric oxide (NO) has been shown to both enhance hydrogen peroxide (H2O2) toxicity and protect cells against H2O2 toxicity. In order to resolve this apparent contradiction, we here studied the effects of NO on H2O2 toxicity in cultured liver endothelial cells over a wide range of NO and H2O2 concentrations. NO was generated by spermine NONOate (SpNO, 0.001–1 mM), H2O2 was generated continuously by glucose/glucose oxidase (GOD, 20–300 U/l), or added as a bolus (200 μM). SpNO concentrations between 0.01 and 0.1 mM provided protection against H2O2-induced cell death. SpNO concentrations >0.1 mM were injurious with low H2O2 concentrations, but protective at high H2O2 concentrations. Protection appeared to be mainly due to inhibition of lipid peroxidation, for which SpNO concentrations as low as 0.01 mM were sufficient. SpNO in high concentration (1 mM) consistently raised H2O2 steady-state levels in line with inhibition of H2O2 degradation. Thus, the overall effect of NO on H2O2 toxicity can be switched within the same cellular model, with protection being predominant at low NO and high H2O2 levels and enhancement being predominant with high NO and low H2O2 levels.