S-nitrosation of protein thiol groups by nitric oxide (NO•) is a widely recognized protein modification. Only few intracellular S-nitrosated proteins have been identified and it has been reported that S-nitrosation/denitrosation can serve as a regulatory process in signal-transduction pathways. Given the potential physiological importance of S-nitrosothiols, and considering that mitochondria are endowed with high levels of thiols and the biochemical requisites for synthesizing NO•, we examined the occurrence of S-nitrosoglutathione (GSNO) in intact, coupled rat liver mitochondria. These organelles contained 0.34nmol of GSNO/mg of protein, detected by HPLC with UV–visible and electrochemical detections. This concentration was dynamically modulated by the availability of NO•; its decay was affected mainly by GSH and superoxide dismutase in a reaction that entailed the generation of GSSG. On the basis of the relatively long half-life of GSNO and the negligible recovery of NO• during its decay, roles for GSNO as a storage and transport molecule for NO• are discussed. Moreover, the formation of GSNO and its reaction with GSH can be considered to be partly responsible for the catabolism of NO• via a complex mechanism that might result in the formation of hydroxylamine, nitrite or nitrous oxide depending upon the availability of oxygen, superoxide dismutase and glutathione. Finally, the high concentrations of GSH in the cytosol and mitochondria might favour the formation of GSNO by reacting with NO• ‘in excess’, thereby avoiding damaging side reactions (such as peroxynitrite formation), and facilitate the inactivation of NO• by generating other nitrogen-related species without the chemical properties characteristic of NO•.
Abbreviations used: GSNO, S-nitrosoglutathione; l-NMMA, NG-monomethyl-l-arginine; NOS, nitric oxide synthase; SOD, superoxide dismutase.