The Superoxide Radical

There are three types of SOD. All SODs have transition metal ions in their reaction center, and the enzymes in both high and low oxidation states react readily (k = 2 x 109 dm3 mol1 s 1 Klug et al. 1972 Fielden et al. 1974 Pick et al. 1974) with 02 [written with Cu as an example reactions (62) and (63)] which allows the enzyme to be recycled. 

The one isolated from bovine blood contains Cu in its reaction center as well as Zn which appears not to take part in the dismutation process. The other two types of SOD contain either Fe or Mn. The CuZn SOD has been found only in eukariotic cells, the Fe SOD only in prokaryotic cells, and the Mn SOD in both (Fee 1981). Aqua-Mn2+ cannot be reduced by 02, but is forms a complex that dismutates giving rise to H202 and 02 (Jacobsen et al. 1997). Such intermediates may also play a role in Mn SOD. 

In neutral solution, (V dominates. The pfCa(I K V) = 4.8 is the selected value (Bielski et al. 1985) from a series of determinations (Czapski and Bielski 1963 Czapski and Dorfman 1964 Sehested et al. 1968 Rabani and Nielsen 1969 Be-har et al. 1970). The rate of self-termination of HCh /CV strongly depends on pH, since only reactions (64) and (65) proceed at an appreciable rate (k64 = 8.6 x 10s dm3 mol1 s 1 k65 = 1.02 X 108 dm3 mol1 s1), while the self-termination of two 02 is too slow to be measurable (k 0.35 dm3 mol1 s 1 Bielski et al. 1985). 

Similarly, nitro blue tetrazolium is reduced by 02 (k = 3 x 104 dm3 mol 1 s 1), and the mono-reduced species subsequently disproportionates yielding the two-electron-reduced monoformazan which absorbs in the visible (e(530) = 2.34 x 103 dm3 mol 1 cm 1 Bielski et al. 1980 for some of the problems that one may encounter using this assay see Cabelli 1997]. 

Except for ET reactions with strong oxidants, 02 is not very reactive (for a compilation of rate constants, see Bielski et al. 1985). For example, practically no reaction has been detected with amino acids (Bielski and Shiue 1979), and there is no reaction to speak of with the DNA constituents, that is, it is also practically unreactive towards DNA. However, where substantial reactivity has been recognized, its main route of reaction seems to be by addition. This has not only been proposed for its reaction with pyrogallol and the propyl ester of gallic acid [k = 3.4 x 10s and 2.6 x 10s dm3 mol1 s 1, respectively cf. reactions (67)-(71) Deeble et al. 1987, 1988], but it seems that an addition reaction triggers a number of chain reactions (von Sonntag et al. 1993, see below). 

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Fridovich, I. (1986). Biological effects of the superoxide radical. Arch. Biochem. 247 1-11. 

A parallel set of determinations was done with Cu2+ added, since this metal ion has been reported to oxidize the superoxide radical ion very rapidly. Thus, with added Cu2+ the first reaction proceeded as shown, but the second was replaced by... 

Walkup, L.K. Kogoma, T. (1989). E. coli proteins inducible by oxidative stress mediated by the superoxide radical. J. Bacteriol. 171, 1476-1484. 

Superoxide dismutase is important for the detoxification of the superoxide radical (O2 ) by reacting with protons to produce H2O2 202 + 2H+ —>62 + H2O2. Although the enzyme generally contains Mn and Fe, or Cu and Zn, the enzyme from Streptomyces seoulensis contains Ni(HI) (Wuerges et al. 2004). 

Absorption of a light quantum leads to an electron-hole pair Eq. (19). The electron reacts with an adsorbed oxygen molecule Eq. (20), and the hole semi-oxidizes a sulfide anion at the surface Eq. (21). Further oxidation of the sulfide anion occurs by O and O2 Eq. (22). The number of Cd ions formed equals that of the sulfate anions The oxidation of illuminated CdS powders was investigated by measuring the consumption and by detecting the superoxide radical,, by an ESR spin trapping method... 

Not all oxidants formed biolc cally have the potential to promote lipid peroxidation. The free radicals superoxide and nitric oxide [or endothelium-derived relaxing aor (EDRF)] are known to be formed in ww but are not able to initiate the peroxidation of lipids (Moncada et tU., 1991). The protonated form of the superoxide radical, the hydroperoxy radical, is capable of initiating lipid peroxidation but its low pili of 4.5 effectively precludes a major contribution under most physiological conditions, although this has been suggested (Aikens and Dix, 1991). Interestingly, the reaction product between nitric oxide and superoxide forms the powerful oxidant peroxynitrite (Equation 2.6) at a rate that is essentially difiiision controlled (Beckman eta/., 1990 Huie and Padmaja, 1993). 

It is well established that aerobes constantly produce small amounts of oxygen-derived species, such as the superoxide radical (02 ), hydrogen peroxide (H2O2) and hypochlorous acid (HOCl), the latter being generated by... 

Aruoma, O.I., Halliwell, B. and Dizdarogju, M. (1989a). Iron ion-dependent modification of bases in DNA by the superoxide radical-generating system hypoxanthine/oxidase. J. Biol. Chem. 264, 13024-13028. 

The mechanism of this electron transfer has been the subject of many studies. Many workers support the involvement of the superoxide radical ion.463Jl6S However, a recent study469 based on EPR470 and electrochemi-... 

All of these uses are based on the behavior of titanium dioxide as a semiconductor. Photons having energies greater than v 3.2 eV (wavelengths shorter than 400 nm) produce electron/hole separation and initiate the photoreactions. Electron spin resonance (esr) studies have demonstrated electron capture by adsorbed oxygen to produce the superoxide radical ion (Scheme 1) (11). Superoxide and the positive hole are key factors in photoreactions involving titanium dioxide reported here are the results of attempts to alter the course of these photoreactions by use of metal ions and to understand better the mechanisms of these photoreactions. 

As the superoxide radical is a precursor of the other reactive oxygen species and interacts with blood plasma components under physiological and pathological conditions as well, systems related to its generation are biologically relevant. It should be noted, however, that with respect to the initiation of lipid peroxidation as one of the main causes of oxidative cell damage, its own reactivity is very weak and that only in protonized form is its toxicity comparable to that of lipid peroxyl radicals [18]. 

Figure 1 shows the graphs of the PCL that were recorded with riboflavin as the photosensitizer and luminol as the detector for free radicals [21], The course of the PCL reaction has two maxima at approximately 30 s and 3 min after the start of irradiation. It has been demonstrated by analysis of kinetics after addition of the reactants at varying times that the first maximum is riboflavin-dependent. Luminol is needed only for visualization of the superoxide radicals. 

In bacteria (Escherichia coli), paraquat is concentrated, reduced to the monocation radical, and combines with molecular oxygen to produce the superoxide radical within the cell. Copper and iron are essential mediators in bactericidal effects. The cytoplasmic membrane is the target organelle in paraquat toxicity to E. coli, and extent of damage correlates positively with levels of these metals (Kohen and Chevion 1988). 

In this case, direct electron transfer between the catalyst and O2 produces the superoxide radical (or other reduced forms of oxygen) which can be involved in a series of subsequent redox reactions. If these reactions are relatively fast, the rate determining step is Eq. (3) and the overall process can be interpreted in terms of relatively simple rate laws. 

However, direct evidence was not presented for the formation of the superoxide radical in the presence of Cu(II) and, as indicated above, the reported observations can be interpreted in terms of the two-electron oxidation model equally well. 

P., Oxidative denitrification of N-omega-hydroxy-L-arginine by the superoxide radical anion, Biochem. J. 317 (1996), p. 17-21... 

The superoxide radical may interact with the protons (resulting, for example, from the reaction described in Equation 12.4), with or without implying other electrons from the conduction band (the species between parentheses may be adsorbed or in the aqueous phase) ... 

Fridovich, I. (1978). The biology of oxygen radicals The superoxide radical is an agent of oxygen toxicity superoxide dismutases provide an important defence. Science 201, 875-80. 

Spin trapping of the superoxide radical anion, as well as that of hydroperoxyl and hydroxyl radicals and related species will be considered later in connection with biological chemistry (pp. 52-54). 

The superoxide anion radical (O2 ) is produced when oxygen accepts one electron. This radical has a short lifetime in aqueous solutions, where it mainly undergoes spontaneous dismutation to hydrogen peroxide and oxygen (Reaction 1). The superoxide radical is in equilibrium with its conjugated acid, the hydroperoxyl radical (HO2), which is a stronger oxidant and generally more reactive than O2... 

Hydroxyl radicals were generated radiolytically in NaO-saturated aqueous solutions of thiourea and tetramethylthiourea. Conductometric detection showed that HO and a dimeric radical cation were produced. The dimeric radical cation is formed by addition of a primary radical to a molecule of thiourea. In basic solution, the dimeric radical cation decays rapidly to a dimeric radical anion, which is formed via neutralization of the cation and subsequent deprotonation of the neutral dimeric radical (Scheme 16). This was not observed in tetramethylurea. These dimeric radical cations of thiourea and tetramethylurea are strong oxidants and readily oxidize the superoxide radical, phenolate ion, and azide ion. 

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