Lack of correlation between the observed stability and pharmacological properties of S-nitroso derivatives of glutathione and cysteine-related peptides.

S-Nitrosothiols (RSNOs) have been widely studied as donors of nitric oxide. In general, RSNOs are considered to be somewhat unstable; however, they are both potent vasodilators and inhibitors of platelet aggregation. In order to improve our understanding of the factors that determine the biological activity of RSNOs, the chemical stability and pharmacological activity of a series of RSNOs was determined. Results show that millimolar solutions of S-nitrosocysteine (SNOCys) and S-nitroso-L-cysteinylglycine (SNOCysGly) were the least stable, whereas S-nitroso-3-mercaptopropionic acid (SNOPROPA) and S-nitroso-N-acetyl-L-cysteine (SNONAC) were the most stable of the compounds tested. Recent evidence suggests that RSNOs, such as SNONAC, are as unstable as SNOCys at micromolar concentrations. The decomposition of certain RSNOs is catalysed by trace amounts of copper (II) ions, with this phenomenon being particularly evident for SNOCys and SNOCysGly. The decomposition of the more stable RSNOs, including S-nitroso-L-glutathione (SNOGSH) and L-gamma-glutamyl-L-cysteine (SNOGluCys), were not as sensitive to copper ions. The decomposition of the stable RSNO, SNOGSH, was more rapid in the presence of excess thiol, whereas the decay of the unstable RSNO, SNOCys, was reduced with added thiol. All RSNOs tested inhibited platelet aggregation, relaxed vascular smooth muscle, and inhibited cell growth in the nanomolar range, but their order of potency did not correlate with their chemical stability of millimolar solutions. It is apparent that the potency of an RSNO in a physiological situation will depend on the concentration of the compound present, the presence of trace metal ions such as copper, and the occurrence of transnitrosation reactions.

Synergism between nitric oxide and hydrogen peroxide in the inhibition of platelet function: the roles of soluble guanylyl cyclase and vasodilator-stimulated phosphoprotein.

In previous studies, a strong synergism between low concentrations of hydrogen peroxide and nitric oxide in the inhibition of agonist-induced platelet aggregation has been established and may be due to enhanced formation of cyclic GMP. In this investigation, hydrogen peroxide and NO had no effect on the activity of pure soluble guanylyl cyclase or its activity in platelet lysates and cytosol. H(2)O(2) was found to increase the phosphorylation of vasodilator-stimulated phosphoprotein (VASP), increasing the amount of the 50-kDa form that results from phosphorylation at serine(157). This occurs both in the presence and in the absence of low concentrations of NO, even at submicromolar concentrations of the peroxide, which alone was not inhibitory to platelets. These actions of H(2)O(2) were inhibited to a large extent by an inhibitor of cyclic AMP-dependent protein kinase, even though H(2)O(2) did not increase cyclic AMP. This inhibitor reversed the inhibition of platelets induced by combinations of NO and H(2)O(2) at low concentrations. The results suggest that the action on VASP may be one site of action of H(2)O(2) but that this event alone does not lead to inhibition of platelets; another unspecified action of NO is required to complete the events required for inhibition.

Use of NO donors in biological systems.

Owing to the increased interest in the biological roles of nitric oxide (NO) the use of NO donors is a desired method of delivering NO to the tissues of interest. This article gives an overview of the most commonly used classes of NO donors and their biotransformation to release NO. A major consideration when choosing an NO donor is the preparation and handling of the compounds. A method has been outlined for the preparation of S-nitrosothiols which eliminates the problem of the overall instability of these compounds both as a solid and in solution. The main aim of this article is to outline the methods used in assessing the ability of NO donors to elicit a biological response in vitro in particular relaxation of vascular smooth muscle and inhibition of platelet aggregation. In addition a method is described for assessing the toxicological potential of NO donors in vitro.