Purification and Properties of Rat Liver Nuclear Protein Kinases

Two protein kinases, designated NI and NII, have been isolated from rat liver nuclei. These enzymes have a similar pH optimum and phosphorylate phosvitin and casein more readily than histone. Both enzymes require magnesium for activity. In the absence of Mg 2+ , other divalent cations such as Ca 2+ , Co 2+ , and Mn 2+ can substitute partially for Mg 2+ when the reaction is catalyzed by NI. With NII, only Co 2+ showed any activity in the absence of Mg 2+ . Magnesium decreased the apparent K m for ATP of protein kinase NI without changing the V max of the reaction, and decreased the apparent K m 's for both ATP and casein, while increasing the V max of the reaction threefold with protein kinase NII. Both enzymes are stimulated about twofold by low concentrations (0.1–0.3 M) of NaCl, KCl, and sodium acetate, whereas higher concentrations (> 0.5 M) inhibit their activities. Both enzymes are inhibited by low concentrations of NaF (0.02 M) and (NH 4 ) 2 SO 4 (0.1 M). NI and NII were found to have sedimentation coefficients of 3.6 S and 10.8 S, respectively. The nuclear protein kinases are not activated by cyclic AMP or cyclic GMP, and are not inhibited by the heat-stable cyclic AMP-dependent protein kinase inhibitor.