Superoxide Anion Reaction

Another way of carrying out electron-transfer mediated oxidation reactions is to use semiconductors as catalysts (Mozzanega et al., 1977). Titanium dioxide will, photocatalyse the oxidation of substituted toluenes to benz-aldehydes by electron transfer from toluene into the photogenerated hole. The electron in the conduction band will reduce oxygen giving the superoxide anion. Reaction of the superoxide anion with the hydrocarbon radical cation produces the aldehyde. A similar mechanism has been used to explain the observation that dealkylation of Rhodamine B (which contains N-ethyl groups) occurs when the dye is irradiated in the presence of cadmium sulphide (Watanabe et al., 1977). 

Luminescence reaction. Pholasin undergoes an oxidative luminescence reaction in the presence of any of the following substances Pholas luciferase, ferrous ions, H2O2, peroxidases, superoxide anions, hypochlorite and other oxidants. In all cases, molecular oxygen is required and pholasin is converted into oxypholasin in the reaction. 

When H2O2 is a necessary component of a luminescence system, it can be removed by catalase. If a luminescence system involves superoxide anion, the light emission can be quenched by destroying O2 with superoxide dismutase (SOD). The ATP cofactor usually present in the fresh extracts of the fireflies and the millipede Luminodesmus can be used up by their spontaneous luminescence reactions, eventually resulting in dark (nonluminous) extracts containing a luciferase or photoprotein. The process is, however, accompanied by a corresponding loss in the amount of luciferin or photoprotein. The use of ATPase and the elimination of Mg2+ in the extract may prevent such a loss. 

NADH, which enters the Krebs cycle. However, during cerebral ischaemia, metabolism becomes anaerobic, which results in a precipitous decrease in tissue pH to below 6.2 (Smith etal., 1986 Vonhanweh etal., 1986). Tissue acidosis can now promote iron-catalysed free-radical reactions via the decompartmentalization of protein-bound iron (Rehncrona etal., 1989). Superoxide anion radical also has the ability to increase the low molecular weight iron pool by releasing iron from ferritin reductively (Thomas etal., 1985). Low molecular weight iron species have been detected in the brain in response to cardiac arrest. The increase in iron coincided with an increase in malondialdehyde (MDA) and conjugated dienes during the recirculation period (Krause et al., 1985 Nayini et al., 1985). 

Indeed, when present in concentrations sufficient to overwhelm normal antioxidant defences, ROS may be the principal mediators of lung injury (Said and Foda, 1989). These species, arising from the sequential one-electron reductions of oxygen, include the superoxide anion radical, hydrogen peroxide, hypochlorous ions and the hydroxyl radical. The latter species is thought to be formed either from superoxide in the ptesence of iron ions (Haber-Weiss reaction Junod, 1986) or from hydrogen peroxide, also catalysed by ferric ions (Fenton catalysis Kennedy et al., 1989). 

The elementary reaction step, which involves the formation of singlet oxygen, is a reaction of superoxide anion radicals CO ), which are the reaction intermediates of the above oxidation when performed in an alkaline medium. In the presence of water they may be converted to hydrogen peroxyl radicals HOO as follows ... 

T. Odajima and I. Yamazaki, Myeloneperoxidase of the leukocyte of normal blood. 3. The reaction of ferric myeloperoxidase with superoxide anion. Biochim. Biophys. Acta. 284, 355-359 (1972). 

V. Massey, S. Strickland, S.G. Mayhew, L.G. Howell, P.C. Engel, R.G. Matthews, M. Schuman, and P.A. Sullivan, Production of superoxide anion radicals in the reaction of reduced flavins and flavopro-teins with molecular oxygen. Biochem. Biophys. Res. Commun. 36, 891-897 (1969). 

Now, we may consider in detail the mechanism of oxygen radical production by mitochondria. There are definite thermodynamic conditions, which regulate one-electron transfer from the electron carriers of mitochondrial respiratory chain to dioxygen these components must have the one-electron reduction potentials more negative than that of dioxygen Eq( 02 /02]) = —0.16 V. As the reduction potentials of components of respiratory chain are changed from 0.320 to +0.380 V, it is obvious that various sources of superoxide production may exist in mitochondria. As already noted earlier, the two main sources of superoxide are present in Complexes I and III of the respiratory chain in both of them, the role of ubiquinone seems to be dominant. Although superoxide may be formed by the one-electron oxidation of ubisemiquinone radical anion (Reaction (1)) [10,22] or even neutral semiquinone radical [9], the efficiency of these ways of superoxide formation in mitochondria is doubtful. 

Section 5.2.3 in Chapter 5. Superoxide dismutase enzymes catalyze dismutation of the superoxide anion radical (O2 ) according to the summary reactions in equation 7.1 ... 

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). 

In this classical Haber-Weiss cycle iron is being reduced by superoxide anion radical (02T), ascorbic acid or glutathione and subsequently decomposes hydrogen peroxide - formed by spontaneous dismutation of 02T - in the Fenton reaction to produce 0H. This iron-driven 0H formation has a stringent requirement for an available iron coordination site, a sine qua non met not only by hexaaquoiron(III) but by most iron chelates (28). Thus, Fe-EDTA, -EGTA, and -ATP retain a reactive coordination site and catalyze the Haber-Weiss cycle. Phytic acid, however, occupies all available iron coordination sites and consequently fails to support 0H generation (Figure 6). 

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