HS-:O2 oxidoreductase 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+ + 02 --> S(0) + H202. At pH 7.8, 20 degrees C, 280 microM 02, and 100 microM HS-, the rate of HS- oxidation is 0.2 mol min-1 (mol SOD)-1. The affinities of the enzyme for HS- and O2 are 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 microM SOD 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.