Mechanism of superoxide dismutase/H₂O₂-mediated nitric oxide release from S-nitrosoglutathione - Role of glutamate

S-Nitrosoglutathione (GSNO), a physiologically relevant nitric oxide (·NO) donor, exhibits antioxidant, anti-ischemic, and antiplatelet properties. The exact mechanism of ·NO release from GSNO in biological systems has not been determined. Both copper ions and copper-containing enzymes have been shown to catalyze ·NO release from GSNO. In this study we observed that copper-zinc superoxide dismutase (Cu,Zn-SOD) in the presence of H₂O₂ caused a rapid decomposition of GSNO, forming oxidized glutathione (GSSG) and ·NO. The cupric ions (Cu²⁺) released from Cu,Zn-SOD were bound to the glutamate moiety of GSNO, yielding a 2:1 (GSNO)₂Cu²⁺ complex. Strong chelators of cupric ions, such as histidine and diethylenetriaminepentaacetic acid, inhibited the formation of (GSNO)₂Cu²⁺ complex, GSSG, and ·NO. GSSG alone inhibited Cu²⁺-induced decomposition of GSNO. This effect is attributed to complexation of copper by GSSG. We conclude that binding of copper to GSNO is obligatory for ·NO release from GSNO; however, the rate of this reaction was considerably slowed due to binding of Cu²⁺ by GSSG. The glutamate moiety in GSNO and GSSG controls copper-catalyzed ·NO release from GSNO, Cu,ZnSOD and H₂O₂ enhanced peroxidation of unsaturated lipid that was inhibited by GSNO. The antioxidant function of GSNO is related to the sequestering of copper by GSNO and its ability to slowly release ·NO. Implications of these findings are discussed in relation to GSNO-induced cardioprotection and to neuropathological processes.