Effect of pH and metal ions on the decomposition rate of S-nitrosocysteine.

S-nitrosothiols (RSNOs) have many biological functions including platelet deactivation, immunosupression, neurotransmission, and host defense. Most of the functions are attributed to nitric oxide (NO) release during S-nitrosothiol decomposition. As the simplest biologically occurring S-nitrosothiol, S-nitrosocysteine (CySNO) has been widely used as an NO donor and has also been incorporated into biomedical polymers. Knowledge of the CySNO decomposition rate is important for assessing the impact of CySNO on various bioengineering applications or biological systems. In this work, spectrophotometer measurements of CySNO decomposition in the presence of metal ions showed that the decomposition rate is highly susceptible to the pH. The maximum decomposition occurs near physiological pH (near 7.4) while in the acidic (pH < 6) and alkaline (pH > 9) condition CySNO is very stable. This demonstrates that blood provides an optimized environment for the decomposition of CySNO leading to the release of NO. The CySNO decomposition rate can also be affected by buffers with different purity levels in the presence and absence of metal ion chelators-although all buffers show the same pH phenomenon of maximizing near physiological pH. An equilibrium model of metal ions as a function of pH provides a plausible explanation for the pH dependence on the experimental decomposition rate.