Regulation of acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine O2-acetyltransferase (lyso-PAF-acetyltransferase) in exocrine glands. Evidence for an activation via phosphorylation by calcium/calmodulin-dependent protein kinase.

Stimulation of secretion in guinea pig exocrine cells is associated with an enhanced synthesis in these cells of 1-O-alkyl-2-sn-acetyl-glycero-3-phosphocholines (PAF) from 1-O-alkyl-sn-glycero-3-phosphocholine (lyso-PAF) (Söling, H-D., and Fest, W. (1986) J. Biol. Chem. 261, 13916-13922). This results from a stimulation of the activity of lyso-1-alkylglycerophosphocholine acetyltransferase (EC 2.3.1.67). Here we have analyzed the effects of various agonists on the activity of this enzyme in guinea pig parotid gland microsomes. Carbamoylcholine leads within less than 30 s to a 2- to 4-fold activation of lyso-PAF-acetyltransferase, which persists after solubilization of the microsomal enzyme with octyl glucoside. The calcium ionophore A23187 has a similar though smaller effect. Neither isoproterenol (2 X 10(-5) M), which stimulates exocytosis more than carbachol, nor phorbol ester significantly affected lyso-PAF-acetyltransferase activity. Incubation of microsomes from unstimulated parotid gland acini with cAMP-dependent and calcium/calmodulin-dependent protein kinase resulted in a 4-fold and 2.9-fold activation of lyso-PAF-acetyltransferase activity, respectively. Protein kinase C had no significant effect. Activation with calcium/calmodulin-dependent protein kinase was inhibited by 40 microM trifluoperazine. When microsomes from carbachol-stimulated glands were used, in vitro activation of the enzyme by calcium/calmodulin-dependent protein kinase was almost abolished. Protein phosphatase 2A in vitro strongly reduced lyso-PAF-acetyltransferase activity in microsomes from both stimulated and unstimulated glands, whereas alkaline phosphatase and protein phosphatase 1 had only small effects. Following treatment with protein phosphatase 2A, enzyme activity in microsomes from stimulated glands could be enhanced more than 8-fold by subsequent incubation with calcium/calmodulin-dependent protein kinase. Although unsuccessful attempts have made it impossible so far to demonstrate directly the incorporation of phosphate into lyso-PAF-acetyltransferase, the results reported here strongly suggest that the enzyme in exocrine cells is regulated by phosphorylation-dephosphorylation and that a calcium/calmodulin-dependent protein kinase is responsible for the activation of the enzyme and type-2 protein phosphatases for its inactivation.