Nitrosation tyrosine

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]. 

Moreover, targeted disruption (knock-down) of the eNOS gene attenuates the neuronal cell death in thiamine-deficient mice (Gibson and Zhang, 2002). eNOS knock-down but not knock-down of iNOS or nNOS leads to a reduction in protein tyrosine nitration (Beauchesne et al., 2009), suggesting a major role of eNOS as the source of nitric oxide-related nitrosative stress in thiamine deficiency. 

Another marker for protein oxidation is nitration of tyrosine residues, and numerous previous studies support the notion that nitrosative stress also contributes to neurodegeneration in AD (Smith et al., 1997 Tohgi et al., 1999 Castegna et al, 2003 Sultana et al., 2006b). A number of mechanisms for tyrosine nitration of protein have been proposed, and the two widely believed to exist in vivo involve formation of peroxynitrite or mediation via hemeperoxidases (Brennan et al., 2002). These mechanisms involve NO or its by-products that react with ROS (Beckman et al. 

Pryor and coworkers have shown that peroxynitrite-mediated nitrosations and nitrations of phenols are modulated by CO2. The reaction was found to be first order with respect to peroxynitrite and zero order with respect to phenol, showing that an activated intermediate of peroxynitrite, perhaps the peroxynitrite anion-C02 adduct (0=N—OO—C02 ), is involved as the intermediate (equation 57) . At pH higher than 8.0, 4-nitrosophenol is the major product, whereas in acidic media significant amounts of the 2- and 4-nitrophenols were formed. Peroxynitrite also induces biological nitration of tyrosine residues of the proteins. The detection of 3-nitrotyrosine is routinely used as an in vivo marker for the production of the cytotoxic species peroxynitrite (ONOO ). It was shown that nitrite anion (N02 ) formed in situ by the reaction of nitric oxide and hypochlorous acid (HOCl) is similarly able to nitrate phenolic substrates such as tyrosine and 4-hydroxyphenylacetic acid . 

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.

In biological systems, the presence of NO may affect proteins via nitration, nitrosation, or nitro-sylation. Nitration involves the addition of NOi. Although NO itself is not an effective nitrating agent, its metabolites like peroxynitrite or the NO2 radical may transform tyrosine residues into nitrotyrosine (Table 4). 

A number of other nitrosations have been studied, often in an environmental or biological context. These include as substrates A -acetylamino acid methyl esters (cysteine, tyrosine, histidine, tryptophan),4-nitrophenol, cimeti-dine, tertiary amines,and methylurea. In the last case an explanation is put forward for the lack of catalytic activity of added bromide or thiocyanate. 

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