Decomposition of S-nitrosothiols: the effects of added thiols

Abstract

The Cu⁺ (added as Cu²⁺) mediated decomposition of the five S-nitrosothiols, derived from penicillamine, cysteamine, thiomalic acid, N-acetylpenicillamine and cysteine have been examined kinetically in the presence of varying amounts of the corresponding thiols. Large differences in behaviour were found. In some cases, reactions were catalysed by added thiols, whereas in others, stability was conferred, often resulting in the appearance of quite large induction effects. The results are explained in terms of the dual function of the thiol (as thiolate), (a) as a reducing agent generating Cu⁺, and (b) as a complexing agent for Cu²⁺, when it is then less available for reduction. The balance of these two effects depends on the structure and concentration of the added thiol. The findings were supported by examining the two effects separately, using ascorbate as a reducing agent, and ethylenediaminetetraacetic acid as the complexing agent. For penicillamine, cysteine and thiomalic acid, the Cu²⁺ complexes were identified from their UV spectra, and their decomposition was followed kinetically.