Superoxide dismutase reaction rate constants

Superoxide radicals are another factor in oxidative damage. They can be determined with nitrobluetetrazolium (NBT), which then forms the colourless formazan. When melanoidins scavenge the superoxide radicals, the colour of the NBT persists.490,491 The activity of a glucose-glycine melanoidin on superoxide radicals is equivalent to the effect of 16 units of superoxide dismutase. The effect of the HMM and LMM fractions of this melanoidin is almost the same. The reaction rate constant of the melanoidin was markedly higher than that of ascorbic acid. If this were due to the reductone structures embedded in the melanodin, it is difficult to explain why the reducing power of the melanoidins is only 0.7 that of ascorbic acid.490... 

Under conditions therefore where 2[Fe(II)] i [(Fe(III)], the efficiency would approach 100%. Where A j [Fe(III)] 3> A 2 [Fe(II)], however, the efficiency would tend to zero. When the metal complexes mimic superoxide dismutase in having equal, high rate constants for Reactions (26) and (27), the efficiency would approach 50%. 

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... 

Using this reaction, McCord and Fridovich evaluated the rate constant for the reaction of superoxide dismutase with Or as approximately 5 X 1011 M"1 sec"1. The corresponding value for the spontaneous disproportionation of Oi- near neutral />H (Eq. (b)) is assumed to be 8.5 x 107 M 1 sec"1 (second-order rate constant) (139, 140). Fridovich et al. concluded from these data that erythrocuprein accelerates the Or disproportionation by 3—4 orders of magnitude. 

The dismutation of O2 by iron superoxide dismutase was found to be similar to that for the copper/zinc bovine superoxide dismutase. The results obtained by Lavelle et al. (1977) showed that catalysis of dismutation of O2 by the iron superoxide dismutase from Photobacterium leiognathi is first order with respect to substrate concentration for all ratios of substrate to enzyme concentrations reported. Although the enzyme is stable between pH 6.0 and 10.0, the value of the rate constant decreases as the pH increases. The second-order rate constant for the reaction be-... 

In contrast the manganese superoxide dismutase appears to exhibit complex kinetics. The simple scheme of one electron reduction and oxidation of the metal appears not to be applicable. Pick et al. (1974) investigating the manganese superoxide dismutase from . coli suggested that not two but four oxidation-reduction reactions are involved and that each reaction is characterized by a different second-rate constant ... 

Superoxide dismutase will scavenge the Of formed and will therefore inhibit the reduction of the dianisidine radical by Of. Consequently the dianisidine radical will dismute to yield the divalently oxidized dianisidine. In the presence of superoxide dismutase this reaction is augmented (Fig. 6). The possibility that Of could reduce the final product of dianisidine oxidation and reverse the change in absorbance at 460 nm was tested and was excluded. The assay has been used to determine the rate constant for purified swordfish liver copper/zinc superoxide dismutase (Bannister et al., 1979) and could be applied to crude extracts. The assay was also found applicable to polyacryalmide gels (Misra and Fridovich, 1977c). Gels soaked in riboflavin plus dianisidine, and subsequently illuminated, developed stable brown bands. Peroxidases are also stained by this procedure due to the photochemical production of hydrogen peroxide. However, the development of bands due to peroxidase activity is much slower than the development of bands due to dismutase activity. 

Pulse radiolysis has met many advantages. The reaction constant can be determined directly and the enzymic activity is linear dependent on the enzyme concentration. The yield of superoxide is extremely high and the method is highly sensitive. The distinction between catalytic and non-catalytic reactions is possible. For example, the reactivity of superoxide dismutase was found to be catalytically and of second order. Rate constants between 1.5 and 2.6x W M s at neutral pH are reported, thus being near the diffusion control By way of contrast, caerulo-... 

Superoxide Dismutase.—Rate constants for the reactions between Oj and the chelates of salicylate, acetylsalicylate, p-aminosalicylate, and di-isopropyl-salicylate at pH 7.5 range from 0.8 to 2.4x 10 M s compared with native cuprein-copper for which A =1.3x 10 M s per g atom of Cu. Thus, these chelates act as perfect model superoxide dismutases. 

Superoxide anion and nitrogen monoxide engender peroxynitrite at almost diffusion-controlled rates (6.7x10 M" . s" Huie and Padmaja 1993 for review see Ducrocq et al. 1999). Its high rate constant means that NO competes effectively with superoxide dismutase for reaction with O2 " (Hogg et al. 1992). 

The rate constants for the free-radical-induced haemolysis of human erythrocytes have been calculated from haemolysis curves and compared with those obtained from experiments with erythrocyte ghosts treated with various radical initiators. The authors concluded that their reaction model (lipid peroxidation/protein oxidation) may be useful in analysing radical-induced haemolysis. In recent years there has been considerable interest in the role the 1-hydroxyethyl radical in the toxic effects of ethanol. Recent studies have shown that the 1-hydroxyethyl radical inactivates the antioxidant enzymes glutathione reductase, glutathione oxidase, and superoxide dismutase. These results indicate that a prolonged generation of the 1-hydroxyethyl radical in animal cells may lower the antioxidant defence status, thus contributing to oxidative stress and toxicity. ... 

Mn(III)TMPyP is a manganese porphyrin that acts as a superoxide dismutase (SOD) mimetic and peroxynitrite decomposition catalyst (Han et al., 2001). SOD mimetics described to date are unstable and are capable of catalyzing undesired side reactions in addition to the dismutation of the superoxide radical. Mn(III)TMPyP is an SOD mimetic with increased stability to pH and hydrogen peroxide. The rate constants for superoxide dismutation and peroxynitrite decomposition are 3.9 X 10 M s and... 

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