Is S-Nitrosocysteine a True Surrogate for Nitric Oxide?

S-Nitrosothiol (RSNO) formation is one manner by which nitric oxide (NO) exerts its biological effects. There are several proposed mechanisms of formation of RSNO in vivo: auto-oxidation of NO, transnitrosation, oxidative nitrosylation, and from dinitrosyliron complexes (DNIC). Both free NO, generated by NO donors, and S-nitrosocysteine (CysNO) are widely used to study NO biology and signaling, including protein S-nitrosation. It is assumed that the cellular effects of both compounds are analogous and indicative of in vivo NO biology. A quantitative comparison was made of formation of DNIC and RSNO, the major NO-derived cellular products. In RAW 264.7 cells, both NO and CysNO were metabolized, leading to rapid intracellular RSNO and DNIC formation. DNIC were the dominant products formed from physiologic NO concentrations, however, and RSNO were the major product from CysNO treatment. Chelatable iron was necessary for DNIC assembly from either NO or CysNO, but not for RSNO formation. These profound differences in RSNO and DNIC formation from NO and CysNO question the use of CysNO as a surrogate for physiologic NO. Researchers designing experiments intended to elucidate the biological signaling mechanisms of NO should be aware of these differences and should consider the biological relevance of the use of exogenous CysNO. Antioxid. Redox Signal. 00, 000–000.