The mechanism of nitric oxide formation from S-nitrosothiols (thionitrites)

Abstract

S-Nitrosothiols (RSNO) are easily made by electrophilic nitrosation of thiols and are a convenient source of nitric oxide. Reaction occurs readily (in many cases) in aqueous buffer at pH 7.4 to give in addition the corresponding disulfide RSSR. If oxygen is not rigorously excluded from the solution, then the nitric oxide is converted quantitatively to nitrite ion, whereas in the absence of oxygen nitric oxide can be detected using a commercial NO-probe. Reaction, however, only occurs (apart from the photochemical pathway) if Cu²⁺ is present. There is often enough Cu²⁺ in the distilled water–buffer components to bring about reaction, but decomposition is halted if Cu²⁺ is complexed with EDTA. Experiments with the specific Cu⁺ chelator neocuproine however show that the true effective reagent is Cu⁺, formed by reduction of Cu²⁺ with thiolate ion. Kinetic experiments show that the most reactive nitrosothiols are those which can coordinate bidentately with Cu⁺, and there is a wide range of reactivity amongst the structures studied. Reactivity is crucially dependent on the concentrations of Cu²⁺ and RS^–.

Reaction also occurs, although somewhat more slowly, if the source of copper is the Cu^II complex with the tripeptide diglycyl-L-histidine (GGH) or as the Cu^II complex with human serum albumin (HSA). This allows the possibility that nitrosothiols could in principle generate nitric oxide in vivo using the naturally occurring sources of Cu^II.

Rapid exchange of the NO-group in RSNO with thiols occurs, again in aqueous buffer at pH 7.4. This reaction has been established as a nucleophilic substitution reaction by the thiolate ion at the nitroso nitrogen atom.

The implications of these results with regard to possible involvement of nitrosothiols in vivo are discussed.