Peroxynitrite oxidative pathways

The different oxidative pathways are possible because tbe 0-0 bond of peroxynitrite can react as if it was either cleaved homolytically into HO + N02 or heterolytically into HO and N02. The third possibility of separating peroxynitrite into HO+ and NO2 is too energetically costly to be considered, requiring 105 20 kcal/mol. Koppenol et al. (1992) have calculated the energetic cost of separating peroxynitrite at pH 7.0 homolytically to be 21 3 kcal/mol, which compares favorably with the experimental activation energy for spontaneous decomposition of 18 2 kcal/mol. The complete separation of peroxy-... 

Three distinct oxidative pathways for peroxynitrite. In addition, peroxynitrite can directly rearrange to nitrate at slightly alkaline pH without acting as an oxidant ( pH 8) (Crow et al., 1994). 

Although hydroxyl radical is commonly assumed to be the most toxic of the oxygen radicals (with little direct evidence), other direct reactions are more likely to be important for understanding the cytotoxicity of peroxynitrite. A second oxidative pathway involves the heterolytic cleavage of peroxynitrite to form a nitronium-like species (N02 ), which is catalyzed hy transition metals (Beckman et al., 1992). Low molecular weight metal complexes as well as metals bound in superoxide dismutase and other proteins catalyze the nitration of a wide range of phenolics, including tyrosine residues in most proteins (Beckman et al., 1992). 

Scorza, G., and Minetti, M., One-electron oxidation pathway of thiols by peroxynitrite in biological fluids Bicarbonate and ascorbate promote the formation of albumin disulphide dimers in human blood plasma. Biochem. J. 329, 405-413 (1998). 

Oxidations by peroxynitrite can take place either directly by ground-state peroxynitrous acid, ONOOH, or indirectly by ONOOH where ONOOH is an activated form of peroxynitrous acid (Goldstein et al. 1996). In the direct oxidation pathway the reaction is first order in peroxynitrite and first order in substrate, and the oxidation yield approaches 100%. In the indirect oxidation pathway the reaction is first order in peroxynitrite and zero order in substrate. In the presence of sufficient concentrations of a substrate that reacts by the indirect oxidation pathway, about 50-60% of the ONOOH directly isomerize to nitric acid, and about 40-50% of the ONOOH is converted to ONOOH. The involvement of hydroxyl radicals in indirect oxidations by peroxynitrite is ruled out on the basis of kinetics and oxidation yields. 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

The existence of nitric oxide synthase (NOS) in phagocytes (see below) provides a different kind of stimulation and the inhibition of NADPH oxidase. It has been found [72] that the low physiological concentrations of peroxynitrite formed from NO and superoxide stimulated superoxide production by PMA-activated human PMNs through the ERK MAPK pathway, while higher peroxynitrite concentrations inhibited it. Moreover, NADPH oxidase was inhibited by lidocaine, a sodium-blocker, in OZ-activated neutrophils through the suppression of p47phox translocation [73]. 

In conclusion, it should be stressed that the competition between pro- and antiapoptotic effects of nitric oxide must probably depends on its relevant levels [137] the low physiological levels of NO principally suppress the apoptotic pathway by several mechanisms, whereas the higher rates of NO production may overcome cellar protective mechanisms and stimulate apoptosis. Furthermore, the simultaneous formation of nitric oxide and superoxide increases the possibility of apoptosis activation due to the formation of peroxynitrite. 

Thus, physiological free radicals superoxide and nitric oxide produced by phagocytes and nonphagocytes are responsible for the two major pathways of LDL oxidation transition metal-dependent and peroxynitrite-dependent mechanisms. However, there is another mode... 

Another pathway of the initiation of lipid peroxidation is the formation of peroxynitrite from superoxide and nitric oxide. Kausalya and Nath [228] found that the FMLP-stimulated... 

The fates of the G(-H) radicals in DNA are mostly determined by reactions with other substrates. Here, we consider the reactions of the G(-H) radicals with types of free radicals that are generated in vivo under conditions of oxidative stress. One of these radicals is the nitrogen dioxide radical, NO2. This radical can be generated in vivo by the oxidation of nitrite, N02, a process that can be mediated by myeloperoxidase [111, 112] as well as by other cellular oxidants [113, 114]. An alternative pathway of the generation of NO2 is the homolysis of peroxynitrite [102, 115] or nitrosoperoxycarbonate formed by the reaction of peroxynitrite with carbon dioxide [99-101]. The redox potential, E°( NO2/NO2")=1.04 V vs NHE [116] is less than that of guanine, E7[G(-H)7G] = 1.29 V vs NHE [8]. Pulse radiolysis [117] and laser flash photolysis [109] experiments have shown that, in agreement with these redox potentials, N02 radicals do not react with intact DNA. However, N02 radicals can oxidize 8-oxo-dG that has a lower redox potential ( 7=0.74 vs NHE [56]) than any of the normal nucleobases [109]. 

The redox potentials of various oxidants derived from nitric oxide and peroxynitrite are summarized in Table 4. Clearly, as the adducts of molecular oxygen and nitric oxide become more reduced, they form substantially stronger oxidizing agents. In effect, addition of one electron makes these nitrogen oxides more ready to accept the next. The precise pathway of decomposition followed is influenced by what types of target molecules come in contact with peroxynitrite and is... 

Trapping of peroxynitrite from rat alveolar macrophages by superoxide dismutase. Although the fotmation of peroxynitrite is drawn as superoxide reacting with nitric oxide in the extracellular space, the actual reactions may be a combination of the pathways shown in Fig. 40. 

The direct reaction of superoxide with nitric oxide is only one of at least four possible pathways that can form peroxynitrite (Fig. 40). For example, superoxide should also efficiently reduce nitrosyldioxyl radical to peroxynitrite. Alternatively, nitric oxide may be reduced to nitroxyl anion, which reacts with oxygen to form peroxynitrite. Superoxide dismutase could even catalyze the formation of peroxynitrite, since reduced (Cu or cuprous) superoxide dismutase can reduce nitric oxide to nitroxyl anion (Murphy and Sies, 1991). Thus, superoxide might first reduce superoxide dismutase to the cuprous form, with nitric oxide reacting with reduced superoxide dismutase to produce nitroxyl anion. A fourth pathway to form peroxynitrite is by the rapid reaction of nitrosonium ion (NO" ) with hydrogen peroxide. This is a convenient synthetic route for experimental studies (Reed et al., 1974), but not likely to be physiologically relevant due to the low concentrations of hydrogen peroxide and the difficulty of oxidizing nitric oxide to nitrosonium ion. 

---