E.p.r. (electron-paramagnetic-resonance), proton-relaxation and u.v.-absorption parameters, and enzyme activity of samples of Cu2+-free bovine superoxide dismutase recombined with different amounts of Cu2+ up to the stoicheiometric [Cu2+]/protein] ratio were investigated after attainment of equilibrium in the recovery process. 2. The e.p.r. spectra were identical with the spectrum of the native protein at all [Cu2+]/[protein] ratios. The relaxation rate of the water protons (T1) and the u.v. absorption increase as linear functions of the added Cu2+. 3. On the other hand, in recombination experiments in the range pH 7.6-10.5 the enzyme activity shows a non-linear increase as the [Cu2+]/[protein] ratio rises. The experimental curves can be interpreted in terms of the model of co-operative binding of Cu2+ to the two sites proposed on the basis of the electrophoretic analyses of the samples, and show that the specific activity of the molecules containing only one Cu2+ ion is twice as high as that of the molecules with two Cu2+ ions. 4. These results support the hypothesis of an anti-co-operative interaction between the two sites during the activity, which allows only one Cu2+ ion to function in catalysis.
