The superoxide dismutase from a radiation-resistant bacterium Deinococcus radiodurans has been purified to electrophoretic homogeneity. The superoxide dismutase has a specific activity of 3300 units/mg and an apparent molecular mass of 43,000 daltons. The enzyme contains 1.5 gram-atom of manganese per mol dimer, and is composed of two identical subunits of 23,500 daltons. The enzyme rapidly loses its catalytic activity and metal content upon dialysis in denaturing reagent, guanidine hydrochloride, and the metal ion chelator 8-hydroxyquinoline. The denatured apoprotein was renatured upon removal of the denaturant by dialysis. The renatured apoprotein assumed a gross conformation similar to the native enzyme as indicated by fluorescence spectroscopy. The renatured apoprotein was reconstituted to the native specific activity upon addition of manganese in the absence of denaturant. The manganese econstituted enzyme contained 1.7 gram-atom of manganese per mol dimer, and had a specific activity of 3650 units/mg. Kinetic studies revealed that the reconstitution with manganese was pH-dependent, and was inhibited by competing metal ions (iron and zinc). / Ph. D.