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) 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 O2 acts as the electron acceptor and is reduced to produce superoxide (O-2). The following experimental evidence is provided in support of this mechanism. Cysteine accelerates the consumption of .NO by 2.5-fold under physiological conditions. The consumption of O2 in the presence of .NO and cysteine is increased by 2.4-fold. The reaction orders of .NO and cysteine are second and first order, respectively. The second order of reaction for .NO may result from interaction between .NO and O-2 to form peroxynitrite. In the presence of Cu,Zn-superoxide dismutase, the reaction of .NO with cysteine generates hydrogen peroxide, indicating that the reaction generates O-2. 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 reacts directly with thiols to form S-nitrosothiol in the presence of an electron acceptor.