Superoxide dismutase turnover rate

Kinetics studies have shown that the rates of the electron-transfer steps are faster than the encounter rate between 02 and the enzyme. The encounter rate is independent of the copper oxidation state. Km for -02 is high (—B.SmM), which means that, under nearly all conditions, superoxide dismutase turnover is far from saturated. 

Rapid freeze epr can be used as a direct assay of superoxide dismutase, as described in the earlier work by Ballou et al., (1969) who studied the effect of superoxide dismutase on the decay of the signal of O2. Superoxide was trapped by rapid freezing during the reaction with oxygen of anaerobically reduced tetraacetyl riboflavin. The O2 available for reaction with superoxide dismutase was ca. 10 M, and under these experimental conditions, it was possible to estimate that the turnover rate number of superoxide dismutase was at least 3 X 10 min . As a consequence of the difficulty inherent in the method, rapid freeze epr has not resulted in routine assay of superoxide dismutase. It has been used in a different approach for mechanistic studies (Fielden et al., 1974). In this case O2 was generated by pulse radiolysis, and the valence state of the enzyme estimated by epr. 

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