Superoxide dismutation activity

Superoxide-dismuting activity of copper rutin complex was confirmed by comparison of the inhibitory effects of this complex and rutin on superoxide production by xanthine oxidase and microsomes (measured via cytochrome c reduction and by lucigenin-amplified CL, respectively) with their effects on microsomal lipid peroxidation [166]. An excellent correlation between the inhibitory effects of both compounds on superoxide production and the formation of TBAR products was found, but at the same time the effect of copper rutin complex was five to nine times higher due to its additional superoxide dismuting capacity. 

The inhibition of lipid peroxidation by metalloporphyrins apparently depends on metal ions because only compounds with transition metals were efficient inhibitors. Therefore, the most probable mechanism of inhibitory effects of metalloporphyrins should be their disuniting activity. Manganese metalloporphyrins seem to be more effective inhibitors than Trolox (/5o = 204 pmol I 1) and rutin (/50 112 pmol I 1), and practically equal to SOD (/50= 15 pmol I 1). The mechanism of inhibitory activity of manganese and zinc metalloporphyrins might be compared with that of copper- and iron-flavonoid complexes [167,168], which exhibited enhanced antiradical properties due to additional superoxide-dismuting activity. 

The mechanisms of superoxide-dismuting activity of SODs are well established. Dismutation of superoxide occurs at copper, manganese, or iron centers of SOD isoenzymes CuZnSOD, MnSOD, or FeSOD. These isoenzymes were isolated from a variety of sources, including humans, animals, microbes, etc. In the case of CuZnSOD, dismutation process consists of two stages the one-electron transfer oxidation of superoxide by cupric form (Reaction (1)) and the one-electron reduction of superoxide by cuprous form (Reaction (2)). 

Y. Ilan, J. Rabani, I. Fridovich, and R.F. Pasternack, Superoxide Dismuting Activity of An Iron Porphyrin, Inorg. Nucl. Chem. Lett., 17 (1981) 93. 

In 1989, we showed [142] that the Fe2+(rutin)2 complex is a more effective inhibitor than rutin of asbestos-induced erythrocyte hemolysis and asbestos-stimulated oxygen radical production by rat peritoneal macrophages. Later on, to evaluate the mechanisms of antioxidant activities of iron rutin and copper-rutin complexes, we compared the effects of these complexes on iron-dependent liposomal and microsomal lipid peroxidation [165], It was found that the iron rutin complex was by two to three times a more efficient inhibitor of liposomal peroxidation than the copper-rutin complex, while the opposite tendency was observed in NADPH-dependent microsomal peroxidation. On the other hand, the copper rutin complex was much more effective than the iron rutin complex in the suppression of microsomal superoxide production, indicating that the copper rutin complex indeed acquired additional SOD-dismuting activity because superoxide is an initiator of NADPH-dependent... 

Thus, the mechanism of MT antioxidant activity might be connected with the possible antioxidant effect of zinc. Zinc is a nontransition metal and therefore, its participation in redox processes is not really expected. The simplest mechanism of zinc antioxidant activity is the competition with transition metal ions capable of initiating free radical-mediated processes. For example, it has recently been shown [342] that zinc inhibited copper- and iron-initiated liposomal peroxidation but had no effect on peroxidative processes initiated by free radicals and peroxynitrite. These findings contradict the earlier results obtained by Coassin et al. [343] who found no inhibitory effects of zinc on microsomal lipid peroxidation in contrast to the inhibitory effects of manganese and cobalt. Yeomans et al. [344] showed that the zinc-histidine complex is able to inhibit copper-induced LDL oxidation, but the antioxidant effect of this complex obviously depended on histidine and not zinc because zinc sulfate was ineffective. We proposed another mode of possible antioxidant effect of zinc [345], It has been found that Zn and Mg aspartates inhibited oxygen radical production by xanthine oxidase, NADPH oxidase, and human blood leukocytes. The antioxidant effect of these salts supposedly was a consequence of the acceleration of spontaneous superoxide dismutation due to increasing medium acidity. 

Many copper(II) complexes, including Cu(DIPS)2 (DIPS = diisopro-pylsalicylate), Cu(salicylate)2, and Cu(Gly-His-Lys), are also active in superoxide dismutation (437, 438), but their use in vivo is limited by dissociation of Cu(II) and binding to natural ligands such as albumin (439). In contrast, the activity of Fe-93 is not affected by albumin (439, 440). 

Unlike Cu(II), free Mn(II) ions are not active in superoxide dismutation, but some Mn(II)-macrocycle complexes catalyze the dismutation of 02 and are biologically active (441). For example, complex 94 (SC-52608) inhibits neutrophil-mediated killing of human aortic endothelial cells in vitro, attenuates inflammation, protects against myo-... 

A superoxide dismutase activity had been reported for the Fe-EDTA complex in contrast with the inactivity of the Cu-EDTA complex. It was shown, on the contrary, that Fe-EDTA, instead of catalysing the dismutation of OJ, interferes with the reduction of nitroblue tetrazolium and of Fe(III)-cytochrome c in the assays of the dismutase activity... 

Superoxide dismutase (SOD, EC 1.15.1.1) is a scavenger of the superoxide anion, and therefore, provides protection against oxidative stress in biological systems [259]. Most SODs are homodimeric metalloenzymes and contain redox active Fe, Ni, Mn or Cu. The superoxide dismutation by SOD is among the fastest enzyme reactions known. The rate constant for CuZnSOD is = 2x 10 s [260], FeSOD is about one order of... 

For molecular electrocatalysts otherwise, and especially transition metal macrocycles, the electrocatalytic activity is often modified by subtle structural and electronic factors spanning the entire mechanistic spectrum, that is, from strict four-electron reduction, as for the much publicized cofacial di-cobalt porphyrin, in which the distance between the Co centers was set at about 4 A [12], to strict two-electron reduction, as in the monomeric (single ring) Co(II) 4,4, 4",4" -tetrasulfophthalo-cyanine (CoTsPc) [20] and Co(II) 5,10,15,20-tetraphenyl porphyrin (CoTPP) [21]. Not surprisingly, nature has evolved highly specific enzymes for oxygen transport, oxygen reduction to water, superoxide dismutation and peroxide decomposition. 

Copper is present in a large number of enzymes, many involved in electron transfer, activation of oxygen and other small molecules like oxides of nitrogen, methane and carbon monoxide, superoxide dismutation, and even, in some invertebrates, oxygen transport (for further details see Granata et al., 2004 Hatcher and Karlin, 2004 Messerschmidt et al., 2001 Rosenzweig and Sazinsky, 2006 Solomon, 2006). 

MnIII/n are much more positive in small molecules than those of FeIII/n [6], the MnSOD must lower the MnIII/n couple to a range suitable for superoxide dismutation while FeSOD raises the Fem/n couple accordingly. Early support for this explanation came from measuring the Em of Fe-substituted MnSOD [Fe(Mn) SOD] in comparison to that of FeSOD. Indeed, the reduction potential of Fe(Mn) SOD drops from the wild-type value of approximately 200 mV to —240 mV (vs. NHE). This report launched extensive study by Miller, Brunold, and others of the mechanisms by which these highly similar proteins tune their active site m [7,8]. 

Fig. 55. Reaction scheme of superoxide dismutation following the bovine enzyme numbering. The first O2 molecule binds to Cu(II) and is stabilized by the H bond to Arg-141. A second superoxide molecule then approaches the active site and, by an outer-sphere electron transfer via the Cu-bound first O2 molecule, reduces the copper to Cud) 44) (step III). Alternatively, O2" directly reduces superoxide to peroxide 336) (step IV), leaving as dioxygen. Note that the Cu(I)-superoxide and Cu(II)-peroxide complexes are resonant forms of the same molecular arrangement. The newly formed peroxide is protonated by Arg-141 and leaves as HO2. Arg-141 receives a proton from the solvent, restoring the active enz5Tne (I). These reaction proposals do not require the breaking and reforming of the Cu-His-61 bridge.

For the superoxide dismutation by a metal complex, it is necessary that the complex redox potential falls between the redox potentials for the reduction and oxidation of O2 (Scheme 3). The potential range between —0.16V (or —0.33V with respect to a standard state of latm O2 pressure the reevaluated standard redox potential of the oxygen/superoxide couple is —0.137V) (28) and -I-0.89V versus NHE is commonly accepted in the literature as the range in which a metal center should exhibit its redox potential for having SOD activity (29). However,... 

That inner-sphere electron transfer plays an important role within the SOD mechanism is shown by our preliminary experiments with the eight-coordinate Mn(II) complex (Fig. 11). Although the redox potential of this complex is similar to the redox potentials of some proven seven-coordinate Mn(II) SOD mimetics (approximately +0.78V vs. NHS) (13a,g,31), the studied eight-coordinate Mn(II) complex demonstrates no ability for catalytic superoxide dismutation. This can be explained in terms of the saturated coordination geometry around the metal center and shows that, for SOD activity, the complex redox potential is not the only important requirement. In the case of these complexes, with a relatively high redox potential, coordination of superoxide is crucial for its efficient reduction. 

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