Peroxynitrite anion formation

The superoxide anion (O2 ) exhibits numerous physiological toxic effects including endothelial cell damage, increased microvascular permeability, formation of chemotactic factors such as leukotriene B4, recruitment of neutrophils at sites of inflammation, lipid peroxidation and oxidation, release of cytokines, DNA singlestrand damage, and formation of peroxynitrite anion (ONOO-), a potent cytotoxic and proinflammatory molecule generated according to equation 7.210 ... 

The precise mechanism by which NO causes glutamase neurotoxicity is unknown. Calcium must be required because of the requirement for NMDA- and glutamate-induced NO formation in brain tissue (Garthwaite etal., 1988). Although both NMDA-receptor agonists and sodium nitroprusside induce specific neurotoxicity as well as cyclic GMP formation in brain tissue (Dawson et al., 1991), it is unlikely that cyclic GMP is the ultimate cause of the neurotoxicity. Instead, NO is most likely involved in producing target cell death. One possible mechanistic pathway is that locally synthesized NO and superoxide anion react with each other to yield peroxynitrite anion (Beckman et al., 1990), which can destroy cell membranes either directly via interaction with cellular thiols (Radi et al., 1991) or indirectly via decomposition to hydroxyl and other free radicals (Beckman et al., 1990). 

The protonated form of peroxynitrite anion, peroxynitrous acid, is highly reactive with biologic molecnles. Hence, the production of nitric oxide from nitric oxide synthase (a complex enzyme containing several cofactors, and a heme group that is part of the catalytic site), which catalyzes the formation of NO from oxygen and arginine, can render ceUnlar components such as DNA susceptible to superoxide-mediated damage (1). 

Reaction 7.24 describes the formation of a new covalent bond between bound NO and 02 (Figure 7.15). Rather than proposing the formation of a Fe(II)-nitrosyldioxyl radical, the DFT computations suggest a two-electron reduction for 02, with the binding of a peroxynitrite anion to Fe(III). 

The recent discovery that nitric oxide (NO) is a signaling molecule ubiquitous in tissue has raised the question that one of the pathways contributing to superoxide toxicity in vivo might be the formation of the highly reactive peroxynitrite anion (ONOO") produced by spontaneous reaction of NO with superoxide (77). It has been shown that perox5mitrite is a substrate of SOD (78). The interaction of SOD with peroxynitrite leads to a permanent modification of the enzyme at Tyr-108. The structural determination of the peroxynitrite-modified Cu2Zn2SOD has been conducted on monoclinic crystals (79). The structure confirms that peroxynitrite permanently modifies the Tyr-108 side chain with formation of 3-nitroty-rosine. The modification does not alter active site residues and the enzyme remains fully active. 

In reaction (24), a new covalent bond forms between bound NO and O2. The product is, according to DFT computations, a peroxynitrite anion bound to Fe(III) the anion has been also proposed as an initial intermediate in the reaction of Mb NO with O2 that subsequently isomerizes to NOs as a final product ( 145). Instead, we proposed the fast bimolecular formation of [Fe(CN)5N02], Equation (25), with a subsequent reaction (26) ... 

Uric acid is present in human plasma at much higher levels than those encountered in other primates because the enzyme urate oxidase is absent from human tissues (Cutler 1984). it has therefor e been proposed that uric acid is an important antioxidant for humans (Ames et al. 1981). Urate reacts with peroxynitrite with an apparent second order rate constant of 4.8 x 10 M s in a complex process, which is accompanied by oxygen consumption and formation of allantoin, alloxan, and urate derived radicals (Santos et al. 1999). The main radical was identified as the aminocarbonyl radical by the electrospray mass spectra of its 5,5-dimethyl-/-pyrroline-N-oxide adduct. Mechanistic studies suggested that urate reacts with peroxynitrous acid and with the radicals generated from its decomposition to form products that can further react with peroxynitrite anion. 

Beckman, J.S., Beckman, T.W., Chen, J., Marshall, P.A. and Freeman, B.A. (1990). Apparent hydroxyl radical formation by peroxynitrite implications for endothelial cell injury from nitric oxide and superoxide anion. Proc. Natl Acad. Sci. USA 87, 1620-1624. 

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. 

Other postulated routes (Jourd heuil et al., 2003) to RSNO formation include the reaction between NO and 02 to yield N02 via a second-order reaction. NO and thiolate anion, RS, react giving rise to thiyl radical, (RS ) [e]. RS then reacts with NO to yield RSNO [f]. The reaction between RS and RS- can also be the source of non-enzymatic generation of superoxide anion (02 ) [g], [h]. 02 reacts with NO to produce peroxynitrite (ONOO ) [i] (Szabo, 2003). Thiols react with ONOOH to form RSNOs [k] (van der Vliet et al.,1998). 

Allopurinol also inhibits reperfusion injury. This injury occurs when organs that either have been transplanted or have had their usual blood perfusion blocked are reperfused with blood or an appropriate buffer solution. The cause of this injury is local formation of free radicals, such as the superoxide anion, the hydroxyl free radical, or peroxynitrite. These substances are strong oxidants and are quite damaging to tissues. 

Several investigators have noted the direct reaction of peroxynitrite with many buffer anions at neutral to alkaline pH (Hughes and Nicklin, 1970 Keith and Powell, 1969). We have found that many common buffer anions, such as formate, will accelerate the decomposition of peroxynitrite (Fig. 28). The mechanism is unknown, but may involve perturbations of water structure by anions. 

The effect of buffer anions on peroxynitrite decomposition. As the concentration of formate is increased, the rate of peroxynitrite decomposition increases but reaches a plateau at high concentrations. The maximum rate depends on the pH. 

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. 

Four routes to form peroxynitrite from nitric oxide. The reaction of nitric oxide with superoxide is only one mechanism leading to the formation of peroxynitrite. Supetoxide could also reduce the nitrosyidioxyl radical. If nitric oxide is directly reduced to nitroxyl anion, it will react with molecular oxygen to form peroxynitrite. At acidic pH, nitrite may form nitrous acid and nitrosonium ion, which reacts with hydrogen peroxide to form peroxynitrite. 

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. 

Figure 11.7. Reactions of nitric oxide involving sulfhydryl groups, a Hypothetical mechanism of nitrosothiol formation. The superoxide anion generated may reaet with a seeond moleeule of NO to yield peroxynitrite. b Migration of nitroso groups between sulfhydryls. This is experimentally illustrated in Figure 11.9b.

Fig. 3 Formation of peroxynitrite from nitric oxide and superoxide anion and reaction products with carbon dioxide...

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