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Protein kinase C catalyzes phosphorylation of the rat skeletal muscle AMP-deaminase in the presence of calcium ions and phosphatidylserine. At the same time, the catalytic subunit of cAMP-dependent protein kinase fails to phosphorylate AMP-deaminase. Ca2+, phosphatidylserine-dependent phosphorylation decreases three-fold (from 0.6 to 0.2 mM) the Km value and does not affect Vmax. Protein kinase C-induced phosphorylation of AMP-deaminase, besides ADP-ribosylation, is suggested to be involved in regulating the AMP-deaminase activity in vivo.

The kinetic properties of cyclic nucleotide phosphodiesterase isolated from the cytoplasmic fraction of lymphoblastoma QOS cells were studied. It was demonstrated that the enzyme can be activated in the presence of micromolar concentrations of cGMP. The kinetic properties of the enzyme are characterized by the nonlinear dependence of the cAMP hydrolysis rate on the substrate concentration. The curve becomes linear in the presence of cGMP. The molecular mass of phosphodiesterase as determined from gel filtration data is 80,0000 Da.

Diol lipids (dioleoyl- and dioctanoylethylene glycol) at relatively low concentrations (approximately 10 microM) were found to activate significantly protein kinase C in the presence of phosphatidylserine or phosphatidylinositol. Since diol lipids are widespread minor lipid constituents of many cells [(1974) Chem. Ind., 597-604], it has been suggested that they may be involved in the maintaining of basal protein kinase C activity in the absence of external stimuli.

Rabbit muscle D-glyceraldehyde-3-phosphate dehydrogenase was shown to serve as a substrate for Ca2+/calmodulin-dependent protein kinase II with a Km of 0.33 microM and a Vmax of 2.63 mumol.min-1.mg-1 at pH 7.5 and 30 degrees C. In the absence of calmodulin, the Vmax was halved and Km unchanged. 0.99 mol of phosphate was incorporated per tetrameric molecule of D-glyceraldehyde-3-phosphate dehydrogenase under the experimental conditions employed.

A modified method for isolation of rat brain Ca2+, phospholipid-dependent protein kinase has been developed. This procedure enables the purification of the enzyme to a state close to a homogeneous one within 18 hours. The first purification step allows for the separation of two peaks of the Ca2+, phospholipid-dependent protein kinase activity; further purification results in a homogeneous enzyme with a Mr of 80 kDa. It was shown that the second peak of protein kinase is a mixture of two polypeptides with Mr of 80 and 74 kDa. The 74 kDa protein also possesses a catalytic activity and is either an isoform or a proteolytic fragment of the native enzyme. The enzyme activation by phosphatidylserine and phosphatidylinositol was studied. It was shown that in contrast to the phosphatidylserine effect, the dependence of the protein kinase activity on phosphatidylinositol concentration is described by two apparent activation constants, which may be suggestive of the existence of two lipid-binding sites in the enzyme molecule: the enzyme affinity for phosphatidylinositol is 4 times higher than for phosphatidylserine. The data obtained suggest that phosphatidylinositol is a physiological activator of Ca2+, phospholipid-dependent protein kinase.

Synapsin I has been isolated from human brain by a rapid and efficient purification technique, and its phosphorylation by human brain Ca2+, phospholipid-dependent protein kinase (protein kinase C) has been studied. The inhibitory effect of calmodulin on this process has been demonstrated. It is also found that non-esterified fatty acids and acidic phospholipids are inhibitory for synapsin I phosphorylation by Ca2+, calmodulin-dependent protein kinase II.