Regulation of CTP:phosphocholine cytidylyltransferase activity and phosphorylation in rat hepatocytes: lack of effect of elevated cAMP levels.

Immunoprecipitation of 32P-labeled CTP:phosphocholine cytidylyltransferase from freshly isolated rat hepatocytes followed by trypsin digestion and two-dimensional peptide mapping revealed multiple phosphorylation sites. Treatment of the hepatocytes with 0.5 mM of the cAMP analog, 8-(4-chlorophenylthio)-adenosine 3':5'-monophosphate or elevation of intracellular cAMP levels by cholera toxin activated the cAMP-dependent protein kinase activity in intact cells. Despite the activation of cAMP-dependent protein kinase no change in the rate of [3H]choline incorporation into phosphatidylcholine was detected. In addition, the activity of cytidylyltransferase in total cell homogenates and its distribution between soluble and particulate fractions remained unchanged. Comparison of peptide maps of 32P-labeled cytidylyltransferase obtained from control and cholera-toxin-treated hepatocytes did not reveal any differences in the phosphorylation state of cytidylyltransferase. Furthermore, only [32P]phosphoserine residues were detected following phosphoamino acid analysis. We conclude that cytidylyltransferase activity is not altered solely by the activation of the cAMP-dependent kinase in fresh hepatocytes.