Effect of Superoxide Dismutase on the Stability ofS-Nitrosothiols

S-Nitrosothiols formed from the nitric oxide (NO)-dependent S-nitrosation of thiol-containing proteins and peptides such as albumin and glutathione (GSH) have been implicated in the transport, storage, and metabolism of NOin vivo. Recent data suggest that certain transition metals enhance the decomposition ofS-nitrosothiolsin vitro. The objective of this study was to determine what effect Cu, Zn superoxide dismutase (CuZn-SOD) has on the stability of certainS-nitrosothiols such asS-nitrosoglutathione (GSNO)in vitro. We found that CuZn-SOD (20 μM) but not Mn-SOD in the presence of GSH catalyzed the decomposition of GSNO with aVmaxof 6.7 ± 0.4 μM/min and aKmof 5.6 ± 0.5 μM at 37°C. Increasing GSH concentrations with respect to CuZn-SOD resulted in complete decomposition of GSNO at concentrations of GSH:SOD of 2:1. Increasing GSH concentrations further from 0.1 to 10 mM resulted in a concentration-dependent attenuation in GSNO decomposition suggesting that SOD-catalyzed decomposition of GSNO would be maximal at concentrations of GSH known to be present in extracellular fluids (e.g., plasma). The decomposition of GSNO by CuZn-SOD resulted in the sustained production of NO. We propose that GSH reduces enzyme-associated Cu2+to Cu1+which mediates the reductive decomposition of theS-nitrosothiol to yield free NO. We conclude that CuZn-SOD may represent an important physiological modulator of steady-state concentrations of low-molecular-weightS-nitrosothiolsin vivo.