Nitration, of proteins

At physiological pH, ONOO- protonates to peroxynitrous acid (ONOOH) which disappears within a few seconds, the end product being largely nitrate. The chemistry of peroxynitrite/peroxynitrous acid is extremely complex, although addition of ONOO to cells and tissues leads to oxidation and nitration of proteins, DNA and lipids with a reactivity that is comparable to that of hydroxyl radicals. 

Aniline, which is used not only to synthesise drugs, pesticides and explosives but also as a building block for materials such as polyurethane foams, rubber, azo dyes, photographic chemicals and varnishes, is manufactured at a quantity of approximately three million tons each year [61]. The toxic effects of aniline include increased nitration of proteins in the spleen [62]. 

Recently, Gunther et al. [41] proposed that nitric oxide may directly react with enzymes without intermediate formation of peroxynitrite. It is known that the oxidation of arachido-nic acid by prostaglandin H oxidase is mediated by the formation of enzyme tyrosyl radical (see Chapter 26). Correspondingly, it has been suggested that NO is able to react with this radical to form the tyrosine iminoxyl radical and then nitrotyrosine. Therefore, the NO-dependent nitration of protein tyrosine residue may occur without the formation of peroxynitrite or other nitrogen oxides. 

Beckman, J. S., Ye, Y. Z., Anderson, P., Chen, j., Accavetti, M. A., Tarpey, M. M., and White, C. R. (1994). Extensive nitration of protein tyrosines observed in human atherosclerosis detected by immunohistochemistry. Biol. Chem. Hoppe-Seyler 375, 81-88. 

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. 

J. S. Beckman, Y. Z. Ye, P. G. Anderson, J. Chen, M. A. Accavitti, M. M. Tarpey, et al.. Extensive Nitration of Protein Tyrosines in Human Atherosclerosis Detected by Immunohistochemistry, Biol Chem Hoppe Seyler 375 (1994) 81-8. 

Archer, S. L., Huang, J. M. C., HampI, V., Nelson, D. P., Shultz, P. J., and Weir, E. K. (1994). Nitric oxide and cGMP cause vasorelaxation by activation of a charybdotoxin-sensitive K channel by cGMP-dependent protein kinase. Proc. Natl. Acad. Set. U.S.A. 91,7583-7587. Beckman, J. S., Ye, Y. Z., Anderson, P. G., Chen, J., Accavitti, M. A., Tarpey, M. M., and White, C. R. (1994). Extensive nitration of protein tyrosines in human atherosclerosis detected by immunohistochemistry. Biol. Chem. Hoppe-Seyler 375, 81-88. Bhagyalakshmi, A., Berthiaume, F., Reich, K. M., and Frangos, J. A. (1992). Fluid shear stress stimulates membrane phospholipid metabolism in cultured human endothelial cells. J. Vase. Res. 29, 443-449. 

Many questions still remain, such as whether extracellular Ap modulates intracellular Ap, or the mechanism by which Ap accumulation leads to synaptic dysfunction. Other factors, such as oxidative stress, which is extensive in AD, may aid the early accumulation of Ap (Butterfield et al., 2(X)2b). AP peptides stimulate oxidative stress by direct and indirect mechanisms. AP-induced oxidative stress may result from an imbalance between reactive oxygen species (ROS) and reactive nitrogen species (RNS), which could react with a number of cellular macromolecular targets including proteins, lipids, carbohydrates, DNA, and RNA. An early marker for oxidative stress is the formation of protein carbonyls, 4-hydroxy-2-tra 5-nonenal (4-HNE) and 3-nitrotyrosme (3-NT), a marker for the nitration of proteins (Butterfield, 2002). Ap peptide can bind to mitochondrial proteins to generate free radicals, or it can promote oxidative stress via neuroinflanunation. Ap peptides stimulate microglial cells to release a neurotoxin, quinoUnic acid, which may also play a role in neurotoxicity (Guillemin et al., 2003). 

Ethylammonium nitrate, CH CH,NH NO , was the first ionic liquid to be discovered. Its melting point of 12°C was reported in 1914 and it has since been used as a nonpolluting solvent for organic reactions and for facilitating the folding of proteins. 

Today microemulsions are used in catalysis, preparation of submicron particles, solar energy conversion, extraction of minerals and protein, detergency and lubrication [58]. Most studies in the field of basic research have dealt with the physical chemistry of the systems themselves and only recently have microemulsions been used as a reaction medium in organic synthesis. The reactions investigated to date include nucleophilic substitution and additions [59], oxidations [59-61], alkylation [62], synthesis of trialkylamines [63], coupling of aryl halides [64], nitration of phenols [65], photoamidation of fluoroolefins [66] and some Diels-Alder reactions. 

In Tobacco. At the time of harvesting, fresh tobacco leaves do not contain measurable amounts of nitrosamines (<5 ppb). However, these compounds are formed during curing, aging and fermentation. Their concentrations depend primarily on the content of proteins, alkaloids, agricultural chemicals and nitrate in the tobacco, as well as on the processing conditions which lead to the reduction of the nitrates. 

Nitration of the tyrosine rings in the four binding pockets of avidin or streptavidin can be done to increase the steric hinderance within the biotin binding sites (Morag et al., 1996). This process yields chromogenic proteins that have reduced binding affinity for biotin, thus allowing elution of biotinylated molecules under mild conditions. 

Arteel and Sies (1999) examined procyanidin oligomers of different size, isolated from the seeds of Theobroma cacao, for their ability to protect against nitration of tyrosine. Serraino and others (2003) investigated antioxidant activity of the blackberry juice and cyanidin-3-O-glucoside on endothelial dysfunction in cells and in vascular rings exposed to peroxynitrite. However, more work is needed in this area, and the confounding effects of oxidized protein/amino acids in the diet need to be elucidated. 

The ability of MPO to catalyze the nitration of tyrosine and tyrosyl residues in proteins has been shown in several studies [241-243]. However, nitrite is a relatively poor nitrating agent, as evident from kinetic studies. Burner et al. [244] measured the rate constants for Reactions (24) and (25) (Table 22.2) and found out that although the oxidation of nitrite by Compound I (Reaction (24)) is a relatively rapid process at physiological pH, the oxidation by Compound II is too slow. Nitrite is a poor substrate for MPO, at the same time, is an efficient inhibitor of its chlorination activity by reducing MPO to inactive Complex II [245]. However, the efficiency of MPO-catalyzing nitration sharply increases in the presence of free tyrosine. It has been suggested [245] that in this case the relatively slow Reaction (26) (k26 = 3.2 x 105 1 mol-1 s 1 [246]) is replaced by rapid reactions of Compounds I and II with tyrosine, which accompanied by the rapid recombination of tyrosyl and N02 radicals with a k2i equal to 3 x 1091 mol-1 s-1 [246]. 

It follows from the above that MPO may catalyze the formation of chlorinated products in media containing chloride ions. Recently, Hazen et al. [172] have shown that the same enzyme catalyzes lipid peroxidation and protein nitration in media containing physiologically relevant levels of nitrite ions. It was found that the interaction of activated monocytes with LDL in the presence of nitrite ions resulted in the nitration of apolipoprotein B-100 tyrosine residues and the generation of lipid peroxidation products 9-hydroxy-10,12-octadecadienoate and 9-hydroxy-10,12-octadecadienoic acid. In this case there might be two mechanisms of MPO catalytic activity. At low rates of nitric oxide flux, the process was inhibited by catalase and MPO inhibitors but not SOD, suggesting the MPO initiation. 

Vasodilation is attributable to nitric oxide (NO), which is produced either directly from the nitroester or liberated by decomposition of NO intermediates (Feelisch and Noack 1987). Either glutathione in cells of vascular tissue or sulfhydryl groups of proteins in these tissues may be responsible for converting nitrates to NO. Nitric oxide activates guanylyl cyclase, which increases intracellular levels of cyclic guanosine 3 5 -monophosphate and thereby produces vasodilation (Kelly and Smith 1996 Robertson and Robertson 1996). 

---