Calcium-activated phospholipid-dependent protein kinases from rat liver: characterization of purified isoenzymic forms.

1. This report describes characteristics of the three isoenzymic forms of protein kinase C purified in our laboratory from rat liver. 2. All three C-kinases phosphorylated several histone preparations, and to a much lesser degree, other protein substrates and had similar Km values for ATP and histones. 3. Each isoenzyme demonstrated an absolute requirement for Ca2+ and negatively charged phospholipids. 4. Among various phospholipids tested, phosphatidylserine from bovine brain was most effective with approximately 220 fold stimulation over basal activity. 5. Both diolein and 12-O-tetradecanoylphorbol-13-acetate decreased the Ca2+ requirement of the isoenzymes and also directly stimulated C-II and C-III enzymes in the presence of suboptimal concentrations of Ca2+ and phosphatidylserine.