Mechanism of Nitric Oxide Release from S-Nitrosothiols*

S-Nitrosothiols have many biological activities and have been suggested to be intermediates in signal transduction. The mechanism and products of S-nitrosothiol decomposition are of great significance to the understanding of nitric oxide (·NO) biochemistry. S-Nitrosothiols are stable compounds at 37°C and pH 7.4 in the presence of transition metal ion chelators. The presence of trace transition metal ions (present in all buffers) stimulates the catalytic breakdown of S-nitrosothiols to ·NO and disulfide. Thiyl radicals are not formed as intermediates in this process. Photolysis of S-nitrosothiols results in the formation of ·NO and disulfide via the intermediacy of thiyl radicals. Reduced metal ion (e.g. Cu+) decomposes S-nitrosothiols more rapidly than oxidized metal ion (e.g. Cu2+) indicating that reducing agents such as glutathione and ascorbate can stimulate decomposition of S-nitrosothiol by chemical reduction of contaminating transition metal ions. Transnitrosation can also stimulate S-nitrosothiol decomposition if the product S-nitrosothiol is more susceptible to transition metal ion-catalyzed decomposition than the parent S-nitrosothiol. Equilibrium constants for the transnitrosation reactions of reduced glutathione, either with S-nitroso-N-acetyl-DL-penicillamine or with S-nitroso-L-cysteine indicate that S-nitrosoglutathione formation is favored. The biological relevance of S-nitrosothiol decomposition is discussed.