Regulation of platelet activating factor synthesis: modulation of 1-alkyl-2-lyso-sn-glycero-3-phosphocholine:acetyl-CoA acetyltransferase by phosphorylation and dephosphorylation in rat spleen microsomes

1-Alkyl-2-lyso-sn-glycero-3-phosphocholine:acetyl-CoA acetyltransferase plays an important regulatory role in the biosynthesis of platelet activating factor, a potent bioactive mediator. We tested the hypothesis that the activity of acetyltransferase may be modulated by enzymatic phosphorylation and dephosphorylation. The results showed that acetyltransferase activity in rat spleens was 2- to 3-fold higher in microsomes isolated in the presence of F-than in those isolated in the presence of Cl-. The microsomal acetyltransferase could be activated by preincubation of microsomes, isolated in the presence of Cl-, with ATP, Mg2+, and the soluble fraction from rat spleen. Addition of phosphatidylserine, diacylglycerols, plus Ca2+ further enhanced the activity. The increase in the activity of acetyltransferase was abolished by treatment of the activated microsomes with alkaline phosphatase. Conversely, the activity of acetyltransferase can be reactivated in the alkaline phosphatase-treated microsomes with incubation conditions that favor phosphorylation. Therefore, our findings suggest that acetyltransferase activity is regulated by reversible activation/inactivation through phosphorylation/dephosphorylation.