The inactivation of superoxide dismutase from bovine and human erythrocytes brought about by ionizing radiation has been studied. Both enzymes have similar radiation sensitivities, but, whereas the solvated electron is almost completely ineffective as an inactivating species, the probabilities of an OH radical or H atom inactivating a native enzyme molecule at pH 7.2 are about 0.04 and 0.1, respectively. H atoms are more effective when the enzyme copper is in the cuprous state, suggesting a conformational difference between the two oxidation states of the enzyme. Inactivation by <tex-math>${\rm Br}_{2}{}^{-}$</tex-math> radicals is negligible at pH 7 but increases sharply above pH 10. The effect of pH upon enzyme inactivation by <tex-math>${\rm Br}_{2}{}^{-}$</tex-math> has been compared with pH effects upon the rate and product absorption spectrum of the <tex-math>${\rm Br}_{2}{}^{-}-\text{enzyme}$</tex-math> reaction. In this way, <tex-math>${\rm Br}_{2}{}^{-}$</tex-math> was shown to inactivate the holoenzyme only when reaction at some histidine residues occurred. The suggestions that, in the native enzyme below pH 10, the copper protects essential histidines from damaging reactions with <tex-math>${\rm Br}_{2}{}^{-}$</tex-math> and that strong copper-histidine interactions are necessary for activity have been supported by studies on the KCN-inhibited and copper-free enzymes.

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