Phosvitin kinase from brain: activation by ions and subcellular distribution

Abstract

The Mg2+ ion-dependent phosvitin kinase prepared from ox-brain grey matter was further activated by a variety of univalent cations. At 200 m[image] concentration, K+ was the most effective cation, but Na+, Rb+, Cs+ and NH4+ ions were only slightly less effective. Choline and tris also stimulated the enzyme system, but Li+ ions were altogether without effect. The percentage stimulation by univalent cations was markedly dependent on the concentration of Mg2+ ions present: for instance, at 1 m[image]-MgCl2, 200 m[image]-KCl increased activity approximately eightfold, whereas at 5 m[image]-Mg Cl2 (the optimum Mg2+ ion concentration in the absence of univalent cation) stimulation was only twofold. Previous reports that this enzyme is stimulated by phosphate ions were not confirmed. GTP was approximately as effective as ATP in donating phosphate to phosvitin. The enzyme was present in the nuclear, mitochon-drial, microsomal and soluble protein fractions of the cell. Both in total activity and activity per unit quantity of protein the soluble proteins were highest. The effect of Na+ and K+ ions on the enzyme of the soluble fraction was compared with their effect on the microsomal enzyme: approximately equal stimulation was found in both cases.