Superoxide anion reaction with nitric oxide

The reaction of nitric oxide with superoxide dismutase is a simple reversible equilibrium, whereas the catalytic cycle with superoxide involves a two step sequence. Consequently, superoxide dismutase may be reduced by superoxide and then react with nitric oxide to form nitroxyl anion. Nitroxyl anion may react with molecular oxygen to form peroxynitrite anion (ONOO"). 

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. 

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.

In addition to oxidants that are generated by the Fenton reaction, superoxide radicals (-02 ) readily react with nitric oxide (NO-), generating peroxynitrite anion (ONOO ) in the following reaction ... 

Fig. 3. Production of reactive species. (A) ROS can be produced from the weak radical oxygen in the mitochondria and endoplasmic reticulum, by various enzymatic reactions, and from oxyhemoglobin. Normally, nontoxic hydrogen peroxide can give rise to the powerful hydroxyl radical in the presence of transition metals (R5). Oxygen can also be induced to react with biomolecules by transition metals and enzymes. RNS can be produced by reaction of superoxide anion radical with the weak radical nitric oxide. These can react to form the powerful oxidant peroxynitrite/peroxynitrous acid, which can cause formation of other radicals, some with longer lives. See the text for details. SOD, superoxide dismutase. (B) Myeloperoxidase in leukocytes can produce the reactive species hypochlorous acid and tyrosyl radical. Unpaired electrons are indicated by the dense dots and paired electrons by the light ones.

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. 

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

However, these indirect effects of nitric oxide derived products are far more prevalent under pathological conditions such as inflammation, where the production of both NO and by the professional phagocytic cell NADPH oxidase enzyme, and induction of iNOS yields the potent cytotoxic species peroxynitrite. Whilst nitric oxide will react with metal centres (as discussed above) at a rate of 5x 10 M" s and the superoxide anion can be dismutated by SOD at a rate of 2.3x10 M s the combined reaction below (Eq. 9), proceeds at a rate faster than either of these individual reactions ... 

The oxidized form of superoxide reductase formed in this reaction is reduced back by rubredoxin, dependent ultimately on reduced pyridine nucleotides via intermediate electron carriers [65]. The reaction of SOR with superoxide is also very fast, the reaction rate constant being of an order of 10 M s. It has been demonstrated that CuZnSOD can also function as superoxide reductase reducing superoxide at the expense of oxidation of ferrocyanide, or as superoxide oxidase, oxidizing superoxide at the expense of reducing ferricyanide [66]. Both ferri- and ferrocyanide are unphysiological substrates but the enzyme can also act as superoxide reductase with nitroxyl anion oxidizing it to nitric oxide [67]. 

Molecular imprinted polymer recognition and on line electrogenerated chemiluminescence detection. Most CL results from a direct oxidation reaction or an oxidation reaction with energy transfer. Commonly used oxidants include hydroperoxide, oxygen, potassium permanganate, ferricyanide, tetravalent cerium ion, lead dioxide and oxygen free radical such as superoxide anion ( 02 ), hydroxy radical ( OH) and nitric oxide (NO). 

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. 

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