Reversibility of cholinephosphotransferase in lung microsomes

The effect of cytidine 5′-monophosphate (CMP) on the incorporation of cytidine 5′-diphosphate (CDP) methyl-¹⁴C]choline or 1-¹⁴C]dipalmitoylglycerol into phosphatidylcholine (PC) catalyzed by rabbit lung microsomal CDP choline:1,2-diacyl-sn-glycerol cholinephosphotransferase (EC 2.7.8.2) was studied. In the presence of 0.85 mM CMP and nonsaturating diacylglycerol concentration, the incorporation of CDP¹⁴C]choline into PC was markedly stimulated, but the incorporation of ¹⁴C]dipalmitoylglycerol into PC was inhibited. This was due to the increase of endogenous diacylglycerol generated from microsomal PC by the cholinephosphotransferase reverse reaction. However, the newly synthesized PC was not readily hydrolyzed in the presence of CMP. The results of this study suggest that the endogenous membranous diacylglycerol is utilized more preferentially for PC synthesis than the exogenous diacylglycerol and that the newly synthesized PC could rapidly equilibrate with the endogenous membrane PC pool.