Copper-zinc superoxide dismutase dismutation reaction

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. 

Lastly the copper oxidases that appear in mammals are listed. Many of these seem similar to their counterparts in the plant kingdom with the exception of some forms of superoxide dismutase. In mammals it is a zinc-copper enzyme and catalyzes the dismutation of the 02" radical according to the reaction Oj" + O2" + 2H" — H2O2 + O2. This reaction... 

Spontaneous dismutation is thus most rapid at the acidic pH values needed to protonate 02% but the rate at neutral or alkaline pH values is greatly accelerated by the presence in chloroplasts of a superoxide dis-mutase enzyme, which catalyzes Reaction 4. Superoxide dismutase in the form of a copper-zinc enzyme is found both free in the stroma and bound to the outside of the chloroplast thylakoids (24-29). 

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-... 

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