S-Nitrosothiols formation

The most convenient route to S-nitrosothiol formation is the nitrosation of thiols by nitrous acid (Eq. 9.13). 

S -nitrosothiols, several of which occur naturally, e.g., iS -nitrosocysteine and S-nitrosoglutathione, have an important role in NO transport and regulation in biological systems. Potential applications of RSNO compounds include their use as vasodilators in the treatment of angina and in the search for a cure for male impotence.11 The most convenient route to S-nitrosothiol formation is the nitrosation of thiols. 

Herold, S., Rock, G., Reactions of deoxy-, oxy-, and methemoglobin with nitrogen monoxide. Mechanistic studies of the S-nitrosothiol formation under different mixing conditions, /. Biol. Chem. 278 (2003), p. 6623-34... 

E., Catalysis of S-nitrosothiols formation by serum albumin the mechanism and implication in vascular control, Proc. Natl. Acad. Sci. USA 99 (2002), p. 5913-5918... 

Byler, D. M., Gosser, D. K., and Susi, H. (1983). Spectroscopic estimation of the extent of S-nitrosothiol formation by nitrite action on sulfhydryl groups. J. Agric. FoodChem. 31, 523-527. 

Stubauer G, Giuffre A, Sarti P (1999) Mechanism of S-nitrosothiol formation and degradation mediated by copper ions. J Biol Chem 274 28128-28133... 

Type II nitrosamines have two reaction pathways. One pathway involves nucleophilic attack at the carbon of C=0 to generate a tetrahedral intermediate which decomposes to an active diazotate ion (R-N=N-0 ). The other pathway involves the nucleophililc attack on the nitrogen of the nitroso group resulting in denitrosation (Scheme 3.5). The nucleophile can be a biologically prevalent thiol, therefore type II compounds are often used as NO donors for the formation of S-nitrosothiols [67, 68]. 

VAN DER VLIET, A., HOEN, P. A., Wong, P. S., Bast, A., Cross, C. E., Formation of S-nitrosothiols via direct nucleophilic nitrosation of thiols by peroxynitrite with elimination of hydrogen peroxide, J. Biol. Chem. 273 (1998), p. 30255-30262... 

DNICs are spontaneously [128] formed in aqueous media using a simple Fe(II) salt, S-nitrosothiol and thiol, with a ratio of Fe2+/RSH of 1 20. NO is transferred quantitatively from the sulfur atom in the RSNO to the iron. The complete mechanism is yet to be fully determined. A 1 2 ratio results in the formation of an EPR silent yellow dinuclear iron complex ([Fe2(RS)2(NO)4]. At the higherer ratio, the green paramagnetic, mononuclear dinitrosyl predominates. The reaction is very straightforward at pH 7.8, under an inert atmosphere and in water. Under anaerobic conditions the stability of this compound is enhanced, however, in the presence of air and hydrogen peroxide, it readily decomposes to give the dinuclear complex [126] which is similar in structure to the Roussin red salt, as shown in Scheme 5.5. 

Compounds of this structure were first described many years ago by a Scottish chemist named MacBeth. He noted that addition of nitrite to an acid solution of a thiol produced a transient red color, which he credited to formation of an S-nitrosothiol or thionitrite. The chemical properties of these compounds have been described elsewhere in this volume by Mutus (Chapter 4). There are also reviews by Williams [11] and by Wang et al. [12]. All known compounds of this class decompose in what appears to be a spontaneous manner as follows ... 

However, a few thiols (cysteine, N-acetylcysteine and thiosalicylic acid) also react with organic nitrates to release NO by another process that is difficult to discern. It could be that the nitrite reacts with a thiol to give an S-nitrosothiol, a ready source of NO, but nitrosation is unlikely to occur at biological pHs. Another possible route to NO involves formation of a thionitrate by trans-esterification [59] (Eq. (12)). This species could then decompose to give NO via an intermediate sulfenyl compound [60] (Eq. (13)). 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

While it is clear that NO is involved in the EDRF, there is also evidence that conjugation with thiols may occur as intermediates [47]. In 1981, Ignarro observed that NO-donating vasodilators react with cysteine to form S-nit-rosocysteine, an activator of guanylate cyclase, and suggested that the formation of unstable S-nitrosothiols was involved in the biological function of nitric oxide... 

Additional studies conducted in this laboratory revealed that NO was responsible for the vascular smooth muscle relaxant effects of several different nitro-vasodilators, and that S-nitrosothiols were intermediates in the intracellular formation of NO (Ignarro et al., 1981). We showed also that NO was a potent inhibitor of human platelet aggregation, and that NO elicited such effects by... 

S-Nitrosothiols undergo a reversible transnitrosation reaction at zinc tris(pyrazolyl)boratozinc thiolates, " TpZn-SR. These zinc thiolates are rmreactive toward anaerobic NO but rapidly react with NO in the presence of O2 or anaerobically with NO2 to release the S-nitrosothiol RSNO with formation of the corresponding zinc nitrate 125). 

The oxidative addition of NO to a thiol, termed S-nitrosation, is a posttranslational modification that can modulate protein function. With high concentrations of NO, this modification can alter protein function indiscriminately however, only a limited number of proteins are S-nitrosated in vivo (8). This selectivity of nitrosothiol formation suggests that a mechanism of regulation of SNO formation and/or decay exists however, the details of this regulation are unknown. 

Chapters 11,18). It binds to the iron of heme groups in either the Fe or Fe " form and also reacts with thiol groups of proteins and small molecules to form S-nitrosothiols (R-S-N=0). It reacts with the heme iron of myoglobin and hemoglobin and, by transfer of one electron, can oxidize the iron of hemoglobin to the Fe + methemoglobin with formation of the nitroxyl ion NO This reaction may be a... 

Guanylate cyclase activation of nitroprusside and nitrosoguanidine is related to formation of S-nitrosothiol intermediates. Biochem. Biophys. Res. Com-mun. 94 93-100... 

A-Acetylcysteine is administered in the ace-toaminophen toxicity. It replenishes the hepatic stores of glutathione (Chapter 17). A-Acetylcysteine is also used in the treatment of pulmonary diseases including cystic fibrosis (Chapter 12). In patients with chronic renal insufficiency, prophylactic oral administration of A-Acetylcysteine have been used in the prevention of further renal impairment due to administration of radiographic contrast agents. In this setting presumably A-Acetylcysteine functions as an antioxidant and augments the vasodilatory effect of nitric oxide via the formation of S-nitrosothiol (Chapter 17). 

P.J. Coupe, D.L.H. Williams (1999). Formation of peroxynitrite from S-nitrosothiols and hydrogen peroxide. J. Chem. Soc., Perkin Trans. 2,1057-1058. 

DNICs have been reported to react with low molecular thiols or protein thiols to yield S-nitrosothiols. Further, DNIC may nitrosate endogenous secondary amines with the formation of nitrosamines [68]. In biological systems, therefore, transnitro-sylation by DNIC would be in competition with nitrosation. This is consistent with findings of Ueno et al. [65] that the efficiency of transnitrosylation of Fe-DTC by DNICs is lower in the liver, a thiol-rich organ, than in the kidney. 

NO in the unbound form has a very short lifetime in the cell but can be stabilized by the formation of complexes, i.e. metal-porphyrin nitrosyls, dinitrosyl-iron complexes and S-nitrosothiols, which are cmisidered to be its biological transporters. Nitric oxide has a very high affinity for iron contained in the active sites of proteins [74]. 

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