Superoxide from oxygen

Tissues are protected from oxygen toxicity caused by the superoxide free radical by the specific enzyme superoxide dismutase. 

Although the two-electron peroxide oxygen transport mechanism appears viable and of some benefit over the four-electron path, the use of superoxide, O J, to provide a one-electron path seems unattained. Gagne [21] has patented such a process, although apparently without actually testing it. It will no doubt be difficult to keep the superoxide from disproportionating ... 

The effectors of the mammalian host immune attack against filaria include reactive oxygen intermediates. Filarial nematodes express glutathione peroxidase, thioredoxin peroxidase and superoxide dismutase at their surface - enzymes believed to protect the nematode from this attack (Selkirk et al., 1998). A bacterial catalase gene has been identified that most probably derives from the endosymbiont genome (Henkle-Duhrsen et al., 1998) this enzyme may contribute with other enzymes to the protection of both Wolbachia and its nematode host from oxygen radicals. 

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

The standard route to the hydroperoxo complexes in Table II is by one-electron chemical (77,78) or electrochemical (74) reduction of the superoxides. From the practical point of view, Ru(NH3)g+ proved to be an especially useful chemical reductant (72,77,78) that reacts rapidly and cleanly according to Eq. (2), and often can be used even in the presence of molecular oxygen. The Ru(NHa)g+ produced in Eq. (2) is quite unreactive and absorbs only weakly in the UV region, causing... 

The key requirement for a SET step in the photocatalytic process seems to be the surface complexation of the substrate, according to an exponential dependence of the probability of electronic tunneling from the distance between the two redox centers [66]. However, as was pointed out in the preceding section on the key role of back reactions, the presence of a SET mechanism could be a disadvantage from an applicative point of view. If the formed SET intermediate (e.g., a radical cation) strongly adsorbs and/or does not transform irreversibly [e.g., by loss of CO from a carboxylic acid or fast reaction with other species (e.g., superoxide or oxygen)], it can act as a recombination center, lowering the overall photon efficiency of the photocatalytic process. 

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). 

The electron transfer from oxygen to the sarcophaginate cation (Reaction (134)) is the rate-determining stage. The next fast step may be either the oxidation of [Co(sep)] + cation with the superoxide radical (Reactions 136 and 137) or disproportionation (Reaction 138). 

Aerobic organisms are protected from oxygen toxicity by three enzymes glutathione peroxidase, catalase, and superoxide dismutase. Superoxide dismutases are metal-loenzymes (the cytoplasmic enzyme contains Cu + and Zn +, whereas the mitochondrial enzyme contains Mn +l widely distributed in aerobic cells. The role of superoxide dismutases in preventing oxygen toxicity is still controversial. For example, some aerobic cells (e.g., adipocytes and some bacteria) lack superoxide dismutase, whereas some strict anaerobes possess this enzyme. 

For example, reactions of xanthine oxidase have been shown to occur by both one-and two-electron mechanisms with oxygen [157,158] and benzoquinone [159] as electron acceptors. Production of superoxide from one-electron donation to oxygen has been demonstrated by rapid-freeze ESR studies [160]. The immobilized free radical species that was detected was identified as Oj by comparison with spectra obtained in chemical systems. However, the fraction of one-electron transfer that occurs depends [157] on a number of factors, including oxygen concentration, pH, and the concentration of the electron donor. The situation with benzoquinone is similarly complex quantitative ESR studies [159] have shown that the extent of one-electron reduction depends upon the concentration of benzoquinone if xanthine is used as donor, but not if NADH is used. In addition, with NADH the reaction is very pH dependent. The apparent Michaelis constant for benzoquinone is much smaller with xanthine than with NADH. Because of the complexities of the xanthine oxidase system, it would appear that data from studies involving acceptors other than oxygen or benzoquinone must be analyzed carefully if reliable conclusions are to be drawn regarding the reaction mechanism. 

Oxygen can be bound to metal ions with several geometries side-on (to form a triangle), end-on linear and end-on bent. The electronic possibiUties include a simple electron-pair donation from oxygen, 1-electron acceptance by oxygen so that there is effectively a coordinated superoxide ion and the charge on the metal is increased one unit (M O2 M 02") and 2-electron acceptance to give effectively coordinated peroxide with the oxidation state of the metal increased by two (M Oj M - 0 )... 

A relatively recent indirect assay was set up by Paoletti (108). The assay consists of a sequence of reactions that generate superoxide from molecular oxygen in the presence of EDTA, manganese(II) chloride, and mercaptoethanol (108-110). The reactions are monitored by following the oxidation of NAD(P)H by superoxide radicals through a decrease of absorbance at 340 nm, which corresponds to the maximum absorbance of NAD(P)H. The addition of SOD (scavenging superoxide) inhibits nucleotide oxidation (Fig. 14). At variance with the previous assays, whereby C3itochrome c is reduced by superoxide, this method relies on the oxidation of NAD(P)H and, in theory, makes the detection less susceptible to aspecific reduction by common cellular components. The assay is very sensitive to the EDTA/Mn ratio and to the mercaptoethanol concentration, both of which affect nucleotide... 

---