HS−:O2Oxidoreductase Activity of Cu,Zn Superoxide Dismutase

Cu,Zn-Superoxide dismutase (SOD, EC 1.15.1.1) catalyzes HS−oxidation according to the reaction: HS−+ H++ O2→ S0+ H2O2. At pH 7.8, 20°C, 280 μMO2, and 100 μMHS−, the rate of HS−oxidation is 0.2 mol min−1(mol SOD)−1. The affinities of the enzyme for HS−and O2are consistent with previous measurements for these substrates in connection with other enzymic reactions. The rate of HS−oxidation by SOD is slower than that provided by other cellular mechanisms, and it is unlikely to be of physiological significance. However, using ≤1 μMSOD the rate of HS−oxidation gradually accelerates so that after 1 h it can be 600 times faster than the initial rate. This is caused by a nonenzymic oxidative chain reaction that is not self-sustaining, but requires “seeding” from the enzymic reaction.