Properties of the cupric sites in bovine superoxide dismutase studied by nuclear-magnetic-relaxation measurements.

Proton nuclear-relaxation rates have been measured as a function of frequency, temperature, pH and cyanide concentration in aqueous solutions of superoxide dismutase from bovine erythrocytes. The results show that, whereas for pH less than or equal to 9 only one water molecule is bound to each Cu2+ ion, at higher pH a second co-ordination site for OH- becomes available; it is proposed that this involves cleavage of the bond between Cu2+ and the histidine residue that bridges to Zn2+.