Mechanism of Nitric Oxide Release from S-Nitrosothiols

Abstract

S-NitrosothiolsS-Nitrosothiols are compounds with the generic structure of RSNO. Under appropriate conditions these compounds decompose to liberate nitric oxide ( ⅐ NO) and the corresponding disulfide (1). It has been suggested that the formation and decay of low molecular weight S-nitrosothiols, such as S-nitrosoglutathione (GSNO) 1 and S-nitrosocysteine (CySNO), may represent a mechanism for the storage or transport of ⅐ NO (2, 3). According to this proposal, S-nitrosothiols are synthesized chemically by reaction of ⅐ NO with thiol. Subsequently, these compounds are transported or diffuse to the site of action. Decomposition of the S-nitrosothiol then leads to ⅐ NO release and the corresponding biological effect. This hypothesis is mainly speculative and remains to be rigorously tested. Little is known about the reaction of ⅐ NO with glutathione (GSH) in vivo; however, the direct reaction of GSH with ⅐ NO does not generate GSNO but forms glutathione disulfide and nitroxyl anion (NO Ϫ ) (4, 5). GSNO is formed only if ⅐ NO is oxidized, by reaction with oxygen, to form ⅐ NO 2 and N 2 O 3 (6). As intracellular oxygen concentrations at the tissue level are in the range of 10 -20 M (7) and as the rate of ⅐ NO oxidation is proportional to the squared power of the ⅐ NO concentration (8), it is likely that the oxidation of ⅐ NO by oxygen in vivo is a slow and insignificant process (4). Evidence for the formation of S-nitrosothiols from endogenous ⅐ NO remains scarce (9). Nevertheless, nitrosylation of protein thiols has been implicated in the ⅐ NO-dependent regulation of many enzymes, including protein kinase C (10) and glyceraldehyde-3-phosphate dehydrogenase (11). It has been reported that normal human serum contains S-nitroso-serum albumin (12, 13) which has been proposed to act as an endogenous regulator of vessel tone (14).Although the physiological relevance of S-nitrosothiols remains to be established, these compounds have been used as donors of ⅐ NO (1, 15, 16). The most commonly employed compounds are GSNO and S-nitroso-N-acetyl-DL-penicillamine (SNAP) (Fig. 1A). Such compounds have been shown to have diverse and remarkable biological effects. For example, SNAP is a potent vasodilator (1) and low concentrations of GSNO have been shown to afford significant protection to the ischemic myocardium (17). It is generally assumed that S-nitrosothiols decompose by homolytic cleavage of the S-N bond (Reaction 1).This process generates ⅐ NO and a thiyl radical, RS ⅐ (18). However, this assumption has not been effectively tested under physiologically relevant conditions. It has been established that S-nitrosothiols are sensitive to both photolytic (19,20) and transition metal ion-dependent breakdown (21) but are stable in the presence of transition metal ion chelators in the dark. The biological activity of S-nitrosothiols may not be exclusively dictated by the release of ⅐ NO as the chemistry of these compounds is complex. S-Nitrosothiols have also been shown to form NO Ϫ , which under appropriate conditions can lead to...

Nitric oxide inhibits insulin-degrading enzyme activity and function through S-nitrosylation

“…We found this to be the case in our experiments, as SNAP inhibited insulin degradation by 50% with no effect on the potency of the compound with the addition of ascorbate. This agrees with the reported mechanism of action of SNAP [39]. In assessing the effect of SNAP on IDE-mediated regulation of proteasome activity, SNAP was found to inhibit proteasome trypsin-and chymotrypsin-like activities with an inhibition curve similar to SNAP inhibition of insulin degradation, which is indicative of NO inhibiting through a similar mechanism.…”

“…SNAP can directly release NO into solution by spontaneous cleavage of its NO moiety or can nitrosylate protein thiols by transfer of NO + [39,40]. The rate of NO release from this nitrosothiol compound is thought to be unaffected by ascorbate.…”

“…The inhibitory effect of PAPA-NONOate on Ab degradation was 25% greater at 1 Â 10 À3 M concentration of compound than the effect of SNAP at the same concentration. This could be attributed to the known half-life of this compound, which is on the order of minutes, compared to the half-life of SNAP, which is a few hours [39,44]. SNP, SNAP, and PAPA-NONOate were not equally efficacious in inhibiting insulin and Ab degradation.…”

How widespread is stable protein S-nitrosylation as an end-effector of protein regulation?

“…However, the stability of S-nitrosothiols is unclear as published half-lives vary dramatically and are clearly condition-dependent [65][66][67][68]. Non-protein Snitrosothiols studied in vitro in the presence of metal chelators are relatively stable with a half-life of S-nitrosocysteine reported at 11 hours [69], although this is decreased to less than 2 minutes when metal ions are present [70], or to less than 20 seconds in the case of cysteine ethyl ester cysteine [16]. The rate of S-nitrosothiols decomposition via transnitrosation is second-order, although rates vary depending on the chemical properties of the thiols involved [66].…”

A potential role for extracellular nitric oxide generation in cGMP‐independent inhibition of human platelet aggregation: biochemical and pharmacological considerations

“…However, studies in platelets are complicated by a number of factors. Firstly, blood plasma contains antioxidants such as ascorbate (Asc; B100 mM; Esteve et al, 1997) and low molecular weight thiols (10-20 mM; Mansoor et al, 1992), which can catalyse the release of NO from S-nitrosothiols (Ignarro et al, 1981;Singh et al, 1996). Secondly, NO and its higher oxides can interact with plasma proteins such as albumin and haemoglobin, resulting in the formation of S-nitrosated proteins with considerably different properties than NO itself (Stamler et al, 1992;Simon et al, 1993;Scharfstein et al, 1994;Kharitonov et al, 1995;Gow et al, 1997;Pawloski et al, 1998;Crane et al, 2002).…”