5- Nitroso thiols

The formation of S-nitroso thiols (R -S-N=0) is partly understood. A hypothetical mechanism is the reduction of A-oxosulfinamide derivatives (9.21, Fig. 9.4), but nothing appears to be known about such a possibility. What has been demonstrated is that nitric oxide by itself does not react with thiols to form 5-nitroso thiols, but does so in the presence of 02. Detailed kinetic analyses led to the mechanism summarized by Eqns. 9.1-9.3 [44] [45], In these and the following reactions, thiols are written as R -SH in consistency with Figs. 9.4 and 9.5. 

Organic nitrites also react with thiols to form 5-nitroso thiols, but, in contrast to organic nitrates, can do so in a single step (Fig. 9.5, Reaction a) [31]. 

This chapter summarizes the current state of knowledge on the role of S-nitroso-thiols in mammalian systems under the following headings Structure and cellular reactivity of RSNOs formation of RSNOs in the biological milieu and physiological role of RSNOs. 

The isolated nitrososulphinates are unstable brown crystals with the N=0 IR absorption band near 1840 cm-1, i.e. at shorter wavelengths than in nitroso thiols (1490-1700 cm-1) due to the powerful electron-withdrawing effect of the sulphonyl group. They decompose... 

S-nitrosated and disulfide derivatives using CZE was developed. S -Nitroso thiols were selectively detected at 320 nm606. 

The release of NO from S-nitroso thiols is a complex process involving a number of factors, in particular cellular factors [46-48]. In a schematic... 

Fig. 9.5. The reaction of organic nitrites with thiols (Reaction a), and a general mechanism for the release of nitric oxide from S-nitroso thiols (Reactions b and c) [31]...

In addition, S-nitroso thiols can act as NO+ donors in S-transnitrosation reactions, which are of great physiological significance, or they can release NO when reacting with thiols [49] [50]. A further mechanism exists for S-ni-trosoglutathione, which is a substrate of glutathione peroxidase. In this case, the enzyme catalyzes the release of NO or a reaction of transnitrosation [51]. 

D. Jourd heuil, K. Hallen, M. Feelisch, M. B. Grisham, Dynamic State of S-Nitroso-thiols in Human Plasma and Whole Blood , Free Radical Biol. Med. 2000, 28, 409-417. 

Glutathionylated proteins can be formed by the interaction of nitric oxide and protein thiols. For example, exposure of mitochondria to NO can lead to the formation of peroxynitrite, an oxidant, that can cause protein glutathionylation. Protein glutathionylation may also occur via the formation of a nitroso thiol protein (PrSNO) followed by the glutathionylate anion displacement of the nitroxyl anion (NO-) by GSH to form protein glutathionylation as shown in Figure 18.13. 

Whether the enzyme action is a result of some type of nitroso thiol formation, the conversion of the N02 to a more reactive nitric oxide free radical (NO°) or other species is not yet established. Figure 10-23 summarizes these possible events. It should be understood that all the pieces in the puzzle are not yet in place. 

The second approach [52] is based on the ability of low-molecular S-nitroso-thiols, namely, S-nitrosoglutathione (GS-NO) and S-nitrosocysteine (Cys-NO), to form DNIC in a direct reaction between Fe " and thiols [53-55] and differs from the former by the ease of performance besides, it excludes purification of gaseous NO from NO2 and the use of buffer solutions. All that is needed for DNIC synthesis is glutathione (cysteine), a bivalent iron salt, sodium nitrite, and distilled water as a solvent... 

Thiophenols in organic solvents (ethanol, benzene, hexane) are rapidly oxidised by NO [55]. The reaction is exothermic and becomes quite hot if the rate of addition of NO is appreciable. A strong green colour develops immediately when the NO comes into contact with the thiophenol solution, and then fades to pale-yellow at the end of the reaction. This colour is due to the S-nitroso thiol. The following scheme of reaction has been proposed ... 

The S-nitroso thiol is an intermediate, and NO is the immediate gaseous product that is formed. The feature of conversions of thiophenols in the presence of NO lies in the recombination of phenylthyil radicals with the formation of disulphide. 

This S-nitroso group is in equilibrium with other S-nitroso compounds formed by reaction of NO with small-molecule thiols such as free cysteine or glutathione (an isoglutamylcysteinylglycine tripeptide) ... 

Nitro- and nitroso-compounds,170171 amines, and thiols induce the decomposition of diacyl peroxides in what may be written as an overall redox reaction. Certain monomers have been reported to cause induced decomposition of BPO. These include AN,172 A -vinylcarbazole,17,177 Ar-vinylimidazole178 and NVP.177... 

The tendency for N-nitrosamides to undergo hydrolysis by a nucleophilic catalysed pathway has been confirmed by studies of N-alkylnitroso acetamides (22) Results summarised in Table I for N -n-butyl-JJ -nitroso acetamide show that its decomposition is also subject to steric constraints (2,6-lutidinestrong nucleophiles (eg. imidazole, thiols) irrespective of their base strength (pK ). Further, the second order dependence on [Imidazole] is more clearly defined for the decomposit-... 

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]. 

Hou et al. developed a method that controlled the generation of a nanomolar amount of NO [173]. A self-assembled monolayer of N-nitroso-N-oxy-p-thiomethyl-benzeamine ammonium salt bound to a gold electrode via a thiol linkage was used for the reaction. When an electric potential was applied, one-electron electrochemical oxidation led to the release of NO (Scheme 3.20). There was a linear relationship between the amount of NO generated and the area of the electrode, indicating that the amount of NO release could be controlled by selecting an appropriately sized... 

This suggests that the attack of the thiolate anion, at least with this product, occurs principally on the 3-position of the furoxan ring. An alternative mechanism to that discussed above was proposed to explain NO-donation by this product. It implies the preliminary cleavage of the 1-2 bond of the furoxan ring, rather than of the 2-3 bond as suggested by Feelisch, to give a tertiary nitroso intermediate. Reasonable mechanisms may be put forward to explain the production of different NO-redox forms from this intermediate [20] (Scheme 6.9). Interestingly, some furoxans, such as 31 and related compounds, produce NO, detected as nitrite, spontaneously without the assistance of thiols [21]. 

Amenates acylated at the exocydic nitrogen are stable as solid compounds but decompose in aqueous solutions releasing NO. This decomposition depends on the pH and most importantly on their chemical structure [147, 152]. The proposed mechanism of NO-release is shown in Scheme 6.24. It is related to that postulated for sydnonimines. The main difference is that here 5-substituted amenates 128 are able to react with water to form acyclic nitroso semicarbazides 129 directly without needing enzymatic cleavage, and these intermediates release NO by an oxidative or thiol mediated mechanism that is not fully understood [153]. 

Due to the electron-demanding carbamoyl substructure on the nitroso hydrazine intermediates 129 the oxidative process that initiates the NO-release is much slower than with the active sydnonimine metabolites. The elimination of HNO from the nitroso intermediates and the subsequent oxidation to NO cannot be completely ruled out for this type of compounds. In vivo, an alternative, possibly thiol mediated route for the NO formation plays a role in the activity [147]. In this reaction the formation of nitrosothiols as unstable NO precursor intermediates is the most likely process. 

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