Biosynthesis of platelet-activating factor (PAF-acether). V. Enhancement of acetyltransferase activity in murine peritoneal cells by calcium ionophore A23187.

The activity of the acetyltransferase capable of transferring the acetyl moiety of acetyl-CoA onto 2-lyso PAF-acether (1-alkyl-sn-glycero-3-phosphocholine) to form PAF-acether was compared in ionophore A23187-stimulated and in non-stimulated rat peritoneal cells. Stimulation resulted in a doubling of the acetyltransferase activity within 30 s. This effect was abolished in the presence of EDTA (1 mM) or EGTA (1 mM) and restored by addition of Ca2+ (10 mM). The specificity of acetyltransferase measured in ionophore-stimulated as well as in untreated cells is the same. In both situations we observed the same Km values for acetyl-CoA, whereas the Vmax values were different. The wide similarities of the two enzyme preparations lead us to conclude that stimulation by the ionophore involves an increase in the number of enzyme molecules rather than a change in the kinetic parameters of the acetyltransferase.