A novel reaction mechanism for the formation of S-nitrosothiol in vivo

The objective of this study was to investigate the mechanism of S-nitrosothiol formation under physiological conditions. A mechanism is proposed by which nitric oxide ((NO)-N-.) reacts directly with reduced thiol to produce a radical intermediate, R-S-N-.-O-H. This intermediate reduces an electron acceptor to produce S-nitrosothiol. Under aerobic conditions O-2 acts as the electron acceptor and is reduced to produce superoxide (O-2(radical anion)). The following experimental evidence is provided in support of this mechanism. Cysteine accelerates the consumption of (NO)-N-. by 2.5-fold under physiological conditions. The consumption of O-2 in the presence of (NO)-N-. and cysteine is increased by 2.4-fold. The reaction orders of (NO)-N-. and cysteine are second and first order, respectively. The second order of reaction for (NO)-N-. may result from interaction between (NO)-N-. and O-2(radical anion) to form peroxynitrite. In the presence of Cu,Zn-superoxide dismutase, the reaction of (NO)-N-. with cysteine generates hydrogen peroxide, indicating that the reaction generates O-2(radical anion). Finally, the formation of S-nitrosothiol is demonstrated in an anaerobic environment and, as predicted by the mechanism, is dependent on the presence of an electron acceptor. These results demonstrate that under physiological conditions (NO)-N-. reacts directly with thiols to form S-nitrosothiol in the presence of an electron acceptor.