Cysteine, nitrosation

We conclude that the major role of bacteria in the nitrosa-tion of dimethylamine is the reduction of nitrate to nitrite and the lowering of the pH of the medium. Furthermore, the complex medium Itself catalyzes nitrosation. The nature of this catalysis is not known, although it could be due to the presence of carbonyl compounds, cysteine, or a variety of other compounds which are known to catalyze nitrosation (17). 

The life cycle of many viruses, including retroviruses, depends on viral proteases that cleave viral glycoproteins into individual polypeptides, and these enzymes are necessary for viral replication. NO can inactivate coxsackievirus [136]. Since cysteine proteases are critical for the virulence and replication of many viruses, nitrosation of viral cysteine proteases may be a mechanism of antiviral host defense. NO mediates nitrosation of cysteine and aspartyl proteases of HIV-1, and it was suggested that this... 

N-Nitrosamines have been shown to be inhibitors of cysteine-containing enzymes. For example, dephostatin and other N-methyl-N-nitrosoanilines (1) were found to be inhibitors of the protein tyrosin phosphatases, papain and caspase [90,91]. Inhibition results from the S-nitrosation of the critical cysteine residues in the active sites of the enzymes by the nitrosamines. Compounds 6 and 7 have been found to inhibit thrombus formation in arterioles and venules of rats [92], while N-nitrosamide 9 exhibited vasodilation and mutagenicity as a result of NO release [93]. 

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)). 

In the absence of added nucleophiles, nitrosation occurs virtually irreversibly by an acid-catalysed pathway, presumably by attack by HjNO or NO". The third order rate constant from the rate equation equivalent to (46) has a value of 840 dm moF s- at 31°C (c/. 456 and 6960 dm mol- s for cysteine and thiourea respectively at 25°C) which suggests that for this neutral substrate the reaction rate is somewhat less than that expected for an encounter-controlled process. There is a major difference between the nitrosation of alcohols and that of thiols in that, whilst the former reactions are reversible (with equilibrium constants around 1), the reactions of thiols are virtually irreversible. It is possible to effect denitrosation of thionitrites but only at high acidity and in the presence of a nitrous acid trap to ensure reversibility (Al-Kaabi et al., 1982). Direct comparisons are not possible, but it is likely that nitrosation at sulphur is much more favoured than reaction at oxygen (by comparison of the reactions of N-acetylpenicillamine and t-butyl alcohol). This is in line with the greater nucleophilicity expected of the sulphur atom in the thiol. For the reverse reaction of denitrosation [(52) and (53)], the acid catalysis observed suggests the intermediacy of the protonated forms... 

NADH peroxidase (Npx EC 1.11.1.1) and NADH oxidase (Nox EC 1.6.99.x) are disulfide oxidoreductases-related enzymes that contain a single redox-active cysteine (16). They supply strictly fermentative bacteria with NAD+ for glycolysis and play an important role in redox signaling in response to oxidative and nitrosative stress (19). 

Interestingly, both NOS and sGC have been shown to be S-nitrosated by low levels of NO (36-38). In NOS, this nitrosation occurs at zinc tetrathiolate cysteines that are critical for maintaining a functional dimer. Modification of these cysteines leads to the formation of inactive monomers, which could be a means of regulating NO production in vivo (37). S-nitrosation of sGC results in the inhibition of NO-stimulated activity (38). This mechanism of desensitization may account for the clinical condition known as NO tolerance, which is an ongoing problem in the treatment of heart disease. 

Many pathways exist to generate a nitrosothiol in vitro by the 1-electron oxidation of NO. Nitrosothiols can be formed via the reaction of a thiol with N2O3, a nitrosating agent that is an intermediate in the decomposition of NO in aerobic solution, or via the direct reaction of NO with a thiol to form an addition complex (SNO ) followed by a 1-electron oxidation. S-nitrosation of a protein thiol also can occur by a trans-S-nitrosation event from a low molecular weight nitrosothiol, such as S -nitrosoglutathione, or from a nitrosated protein cysteine (8). Whereas the in vivo mechanism of protein S -nitrosation is unknown, a protein-mediated trans-S -nitrosation mechanism is an attractive possibility because of the specificity it could impart on the reaction. Additionally, the same protein could catalyze both the nitrosation and denitrosation of a specific cysteine. A report showing that the protein thioredoxin can transnitrosate caspase-3 selectively and reversibly lends support to this proposal (34). 

Mitchell DA, Marietta MA. Thioredoxin catalyzes the S-nitrosation of the caspase-3 active site cysteine. Nat. Chem. 

ONOO can react with cysteine residues via oxidative, nitrosative and nitro-sylation to yield disulphides and nitrosothiols. For glutathione (GSH), the main NO mediated nitrosative pathway has been elucidated as described by Eqs. 21 to Eq. 23 ... 

The nitrosation of -amino acids is especially interesting because of the biological importance of N-nitrosoamines (see Sect. 4.2). The nitrosation mechanism was investigated first with amino acids containing a secondary amino group, namely proline (4.1, X = H), 4-hydroxyproline (4.1, X = OH), and sarcosine (4.2), but also with cysteine (4.3). 

Fig. 1 Posttranslational redox modificatitnis to amino acids in proteins. Many amino acids can undergo various posttranslatiraial redox modifications in the presence of NAPQI, oxidative stress, and nitrosative stress. Thiols in cysteine can undergo covalent binding, mixed disulfide formation, nitrosylation, and become oxidized to sulfenic, sulfinic, and sulfonic acids. Tyrosine can become nitrated by peroxynitrate, and methionine can be oxidized by ROS to methionine sulfoxide. Not shown are many other oxidatirais that can occur to other amino acids such as proline, histidine, etc.

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