Metabolism of Snitrosothiols

Cleavage of the S—N bond in S-nitrosothiols can occur homolytically or heterolytically, thereby yielding NO, NO+, or NO-, depending on redox conditions. Homolytic cleavage of the S—NO bond photochemically was first shown in 1966 [47]. This photolytic cleavage is likely responsible, at least in part, for the action spectrum of vascular tissue, and supports the importance of S-nitrosothiol formation in vivo. Furchgott and colleagues first showed that light can induce relaxation of blood vessels [48], which is a consequence of release of nitric oxide from a photolabile store [49] and requires adequate glutathione stores [50-52]. Homolytic cleavage of the S—NO bond by thermal decomposition has also been reported [53]. Recent data, however, suggest that this mechanism of decomposition is not relevant under physiological conditions [54]. In plasma, NO release from S-NO-albumin and S-NO-glutathione may be regulated by heterolytic NO+ transfer and reductive activation to NO, rather than by homolytic decomposition of labile S-nitrosothiols [55].

GSNO is the model compound used for most studies of S-nitrosothiol metabolism. GSNO is fairly stable under physiological conditions in plasma and buffer solutions, while other low-molecular-weight S-nitrosothiols have tissue half-lives ranging from seconds to minutes, much shorter than protein S-nitrosothiols [25]. More information on the relevant reaction pathways and kinetic constants is given in Chapter 9. The ability of the tissue homogenate supernatants to facilitate decomposition of GSNO remains unchanged after removal of proteins and other high-molecular-weight compounds [56], suggesting that the decomposition rate of GSNO is determined by low-molecular-weight compounds; other studies suggest that enzymatic catalysis is involved in GSNO metabolism [14,19,20,57-59].

Superoxide anion [60] and transition metal ions [33,38,61-63] were also found to accelerate greatly S-nitrosothiol decomposition in vitro; interestingly, these factors also facilitate its formation. Reduced metal ions (e.g., Cu+) decompose S-nitrosothiols more rapidly than oxidized metal ions (e.g., Cu2+), indicating that reducing agents such as glutathione can stimulate decomposition of S-nitrosothiol by chemical reduction of contaminating transition metal ions [64,65]. CuZn-SOD, but not Mn-SOD, catalyzes the decomposition of GSNO and the formation of NO in the presence of GSH at concentrations present in extracellular fluids [66].

Ascorbate exists in cellular systems at relatively high concentration, and is a very important cellular antioxidant. Decomposition rates of S-nitrosothiols in the presence of ascorbate [67] increase drastically with increasing pH, signifying that the most highly ionized form of ascorbate is the more reactive species [68]. The reaction is also accelerated in the presence of trace redox-active copper [69]. Ascorbate has been shown to block S-nitrosation of proteins [25,31,70-72], and in plasma and tissue, ascorbate is one of the critical determinants of the overall S-nitrosothiol pool [55,73]. The vasodilator activity of GSNO is significantly enhanced by ascorbic acid [69]. Ascorbate has also been shown to be necessary for transformation of organic nitrates to nitric oxide by xanthine oxidase, where it releases NO from S-nitrosothiol intermediates [44].

Despite these findings, however, there are only a few studies on the cellular metabolism of these compounds. Cells consume GSNO at a much greater rate than the spontaneous GSNO decomposition rate, which depends on protein thiols [74,75]. Gordge suggested that this active decomposition of GSNO is mediated by "GSNO lyase" [59]. One candidate enzyme for this activity is y-glutamyltranspeptidase, which accelerates the decomposition of GSNO by hydrolyzing the glutamyl moiety to yield S-nitrosocysteinylglycine (CG-SNO). The latter is susceptible to transition metal ion-dependent decomposition and release of NO [75]. A glutathione-dependent formaldehyde dehydrogenase has recently been identified as a major protein responsible for GSNO-metabolism [58], viz., the S-nitrosoglutathione reductase activity [76], in eukaryotes. This protein is conserved across phyla [77], and was renamed GSNO reductase (GSNOR). GSNOR has been shown to protect yeast cells against nitrosative stress both in vitro [58] and in vivo [57]. In a GSNOR-deficient mouse, S-nitrosothiols were markedly increased, and the liver, immune system, and cardiovascular system manifested S-nitrosative stress [14] under these conditions. The effect of GSH, the substrate for GSNOR, in the metabolism of S-nitrosothiols is still debatable. Xu and colleagues suggested that GSH shortens GSNO half-life [69], while other studies show that buthionine sulfoximine treatment, which decreases intracellular GSH levels, does not affect GSNO decay [59,74].

It is not yet known whether there is any enzyme that can metabolize a protein S-nitrosothiol directly. Thioredoxin or thioredoxin reductase may be one candidate, which has been shown to restore S-nitrosation-induced inhibition of protein kinase C activity in pulmonary endothelial cells [78].

0 0

Post a comment