Mechanistic studies of copper and thiolate ion induced S-nitrosothiol decompositions
A detailed study concerning the aqueous decomposition characteristics of S-nitrosothiols in both the presence and absence of cupric ions was undertaken. Spectrophotometric measurements established that the true catalytic species generating nitric oxide from S-nitrosothiols is Cu(^+), formed by the reduction of copper(II) ions by thiolate, which is present as an impurity in solution. Introduction of the specific cuprous ion chelator neocuproine inhibited reaction, with the concentration of thiol in situ having a significant influence on the absorbance/time traces obtained. Under certain conditions thiolate ions clearly promoted S-nitrosothiol decomposition, whereas at times an opposite effect was noted. These results have been correlated with the reductive ability and chelation properties towards Cu(^2+) of each thiol in question. Structure/reactivity studies were extended further to include a range of S-nitrosated aromatic and heterocyclic thiols which generated the corresponding disulfides in distilled water yet reformed the appropriate thione at pH 7.4, along with nitric oxide in both media. A mechanism has been proposed which accounts for these observations. The reaction of S-nitrosothiols with cupric ions bound to biologically significant molecules such as amino acids, peptides and proteins was followed. Despite Cu(^2+) being chelated in this manner, S-nitrosothiol decomposition was apparent, albeit at a slower rate than that seen when copper(II) sulfate pentahydrate was utilised. Thiolate ions were capable of reducing Cu(^2+) Cu(^+) which was bound to such molecules suggesting a possible mechanism for nitric oxide formation from S-nitrosothiols in vivo. The blue copper protein ceruloplasmin also promoted NO generation under physiological conditions. A brief investigation into the direct reaction of thiolate ion with its corresponding S-nitrosothiol was also carried out. It was discovered that the major reaction product in this instance is ammonia and not nitric oxide, suggesting that a different copper-ion independent process is occurring involving direct interaction between the two species.