Structure-reactivity studies of the Cu(2+)-catalyzed decomposition of four S-nitrosothiols based around the...

S-Nitrosoglutathione (GSNO) and the dipeptide derivative S-nitrosoglutamylcysteine (SNO-GluCys) both at 1 x 10(-3) M in pH 7. 4 buffer containing added Cu(2+) (1 x 10(-5) M) are very unreactive toward decomposition (measured spectrophotometrically), and in both cases reaction stops at very low conversion. S-Nitrosocysteine (SNC) and the dipeptide derivative S-nitrosocysteinylglycine (SNO-CysGly), on the other hand, are orders of magnitude more reactive under the same conditions, and reaction proceeds to completion. Initially, we interpreted these results in terms of the requirement of a suitably positioned free NH(2) group (which is available in both SNC and SNO-CysGly, but not in GSNO and SNO-GluCys) for efficient complexation of Cu(+), the effective reagent. However, later results measured at much lower substrate concentration (1 x 10(-6) M) using the NO electrode system showed that at this concentration, all four S-nitrosothiols react at approximately the same rate and yield NO quantitatively. For GSNO the rate and percentage conversion were shown to drop progressively as the substrate concentration increases. All reactions are effectively halted in the presence of the metal ion chelator EDTA. The results can readily be explained in terms of complexation of Cu(2+) by the product disulfides from GSNO (i.e., GSSG) and SNO-GluCys, involving the glutamate residue, which is not present in SNC and SNO-CysGly. This is confirmed by the observed progressive reduction in yield and percentage conversion of GSNO decomposition as GSSG is added, at micromolar substrate concentrations.