Reactions of hydrogen peroxide with superoxide dismutase from Propionibacterium shermanii--an enzyme which is equally active with iron or manganese--are independent of the prosthetic metal.

Propionibacterium shermanii contains a single constitutive superoxide dismutase (SOD) which is active with either iron or manganese incorporated in the same protein moiety. Copper and cobalt can also be incorporated by the bacteria in the active center of the SOD under conditions of metal deficiency, but in this case the enzyme is enzymatically inactive. In contrast to other bacterial SODs, the Fe-SOD of P. shermanii remains highly resistant to inactivation by hydrogen peroxide, as does Mn-SOD. Both SOD types cannot be distinguished by their inactivation patterns. Incubation with hydrogen peroxide results in a concentration- and time-dependent decrease in tryptophan fluorescence, independent of the metal present in the active center. Moreover, the Fe-SOD shows a time-dependent decrease in spin concentration after addition of hydrogen peroxide, which reflects alterations in the environment of the metal rather than a reduction of Fe3+ to Fe2+. No obvious correlations exist, however, between these effects and the enzymatic activity of the enzyme. The resistance of the SODs from P. shermanii to inactivation by hydrogen peroxide seems to be caused by the fact that a tryptophan residue near the metal-chelating histidine-75--which is present in all Fe-SODs being rapidly inactivated by this agent--is exchanged for valine.