Hydrogen peroxide dismutation

III) form of Lactobacillus plantarum is depicted in Fig. 13 [105,106]. The Mn centers are bridged by oxo, hydroxo, and car-boxylato ligands in a manner similar to the binuclear structures (8-10) in Sect. 16.1.4. A proposed mechanism for hydrogen peroxide dismutation by Mn catalase is presented in Fig. 14. A number of model studies of H2O2 disproportionation employing bridged binuclear Mn complexes have been conducted [107]. 

Classical tetrameric heme-catalases are highly efficient catalysts for hydrogen peroxide dismutation to dioxygen and water at a very high rate of 10 cycles s ... 

Bettelheim, A., D. Ozer, R. Harth, and R. Ydgar (1990). Dioxygen reduction and hydrogen peroxide dismutation using electropolymerized bilayers of cobalt -1- manganese tetrakis(o-aminophenyl)porphyrins./. SecfroanaZ. Chem. 2S1, 147-161. 

The above reaction is followed by the spontaneous production (by spontaneous dismutation) of hydrogen peroxide from two molecules of superoxide ... 

Thus, superoxide itself is obviously too inert to be a direct initiator of lipid peroxidation. However, it may be converted into some reactive species in superoxide-dependent oxidative processes. It has been suggested that superoxide can initiate lipid peroxidation by reducing ferric into ferrous iron, which is able to catalyze the formation of free hydroxyl radicals via the Fenton reaction. The possibility of hydroxyl-initiated lipid peroxidation was considered in earlier studies. For example, Lai and Piette [8] identified hydroxyl radicals in NADPH-dependent microsomal lipid peroxidation by EPR spectroscopy using the spin-trapping agents DMPO and phenyl-tcrt-butylnitrone. They proposed that hydroxyl radicals are generated by the Fenton reaction between ferrous ions and hydrogen peroxide formed by the dismutation of superoxide. Later on, the formation of hydroxyl radicals was shown in the oxidation of NADPH catalyzed by microsomal NADPH-cytochrome P-450 reductase [9,10]. 

Vitamin A (retinol) is not a classic antioxidant although it is frequently related to a group of antioxidant vitamins E, C, and A. Murata and Kawanishi [83] found that retinol and its derivative retinal induced the formation of 8-HOdG in HL-60 cells. This process was supposedly mediated by hydroxyl radicals formed from hydrogen peroxide (the product of superoxide dismutation) and endogenous transition metal ions. 

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). 

ROS generation is generally a cascade of reactions that starts with the production of superoxide. Superoxide rapidly dismutates to hydrogen peroxide either spontaneously, particularly at low pH, or catalyzed by superoxide dismutase (SOD). 

The hydroperoxyl radical formed, "00H, has a pKg of 4.75 and exists in water as a superoxide radical anion 02 Either species in water dismutates to produce O2 and hydrogen peroxide, H2O2, or, possibly, react with dissolved or adsorbed phenol. Dilute H2O2 does... 

Stopped-flow measurements with superoxide in aqueous solution at physiological pH are not possible due to its fast self-dismutation under these conditions. Therefore, the indirect assays such as McCord-Fridovich, adrenalin and nitroblue tetrazolium (NET) assays are widely used in the literature, not only for qualitative but also for quantitative detection of SOD activity of small molecular weight mimetics 52). Not going into details, we just want to stress that the indirect assays have very poor even qualitative reliability, since they can demonstrate the SOD activity of the complexes which does not react with superoxide at all. It has been reported in the literature that this is caused by the interference of hydrogen peroxide 29). We have observed that the direct reaction between complexes and indicator... 

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... 

Fig. 2. The Bonnichsen, Chance, and Theorell 34) mechanism for the dismutation of hydrogen peroxide by catalase. (A) The simple ping-pong mechanism (ferric-peroxide compound (ycle) involves only the successive formation and decomposition of the compound 1 intermediate by two successive molecules of H2O2. (B) Reversible ES(Fe -H202) and ternary (compound I-H2O2]) complexes are added to the mechanism in A.

Manganese is used by nature to catalyze a number of important biological reactions that include the dismutation of superoxide radicals, the decomposition of hydrogen peroxide, and the oxidation of water to dioxygen. The dinuclear manganese centers that occur in Lactobacillus plantar-aum catalase and Thermus thermophilus catalase have attracted considerable attention and many model compounds have now been synthesized that attempt to mimic aspects of these biological systems.The catalases have at least four accessible oxidation states (Mn Mn , Mn°Mn , Mn" Mn", and Mn Mn ) it is believed that the Mn"Mn"/Mn"Mn" redox couple is effective in catalyzing the disproportionation of water. 

In part motivated by the desire to model biological redox processes, there have been many studies in which Robson-type macrocycles (205) (R = H) have been employed to form dinuclear manganese species.For example, a novel macrocyclic heterodinuclear catalase-like model complex of type (206) has been reported. " This complex can dismute hydrogen peroxide to dioxygen in basic aqueous solution. 

Bagchi and Kleiner suggested that endogenous HA can undergo dismutation by the action of a cyanobacterial hydroxylamine dismutase, thereby providing a detoxification mechanism for excess hydrogen peroxide, as well as for an excess of HA itself. This... 

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