Nitric oxide protects Cu, Zn-superoxide dismutase from hydrogen peroxide- induced inactivation

Reaction of Cu, Zn-superoxide dismutase (SOD1) and hydrogen peroxide generates a putative oxidant SOD-Cu²⁺-(·)OH that can inactivate the enzyme and oxidize 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) to DMPO-(·)OH. In the presence of nitric oxide ((·)NO), the SOD1/H₂O₂ system is known to produce peroxynitrite (ONOO^-). In contrast to the proposed cytotoxicity of (·)NO conferred by ONOO^-, we report here a protective role of (·)NO in the H₂O₂-induced inactivation of SOD1. In a dose-dependent manner, (·)NO suppressed formation of DMPO-(·)OH and inactivation of the enzyme. Fragmentation of the enzyme was not affected by (·)NO. Bicarbonate retarded formation of ONOO^-, suggesting that (·)NO competes with bicarbonate for the oxidant SOD-Cu²⁺-(·)OH. We propose that (·)NO protects SOD1 from H₂O₂- induced inactivation by reducing SOD-Cu²⁺-(·)OH to the active SOD-Cu²⁺ with concomitant production of NO⁺ which reacts with H₂O₂ to give ONOO^-.