Formation of alkylacyl- and diacylglycerophosphocholines via diradylglycerol cholinephosphotransferase in rat liver.

We found that diacylglycerol and alkylacylglycerol choline phosphotransferase activities exhibited similar pH optima, thermolabilities and inhibitions by Mn2+, and dithiothreitol. The Vmax of diacylglycerol cholinephosphotransferase was higher (approx. 1-2-fold) than the Vmax of alkylacylglycerol cholinephosphotransferase. The Km value for diacylglycerol was somewhat greater than for the alkylacylglycerol, but no differences were found for the Km of CDPcholine with either type of diradylglycerol as the other substrate. Endogenous levels of diacylglycerols in microsomes were 24.3 nmol/mg protein, whereas no detectable amount of alkylacylglycerols was observed. Results from this study indicate that a similar or identical enzyme catalyzes the formation of 1,2-diacyl- and 1-alkyl-2-acyl-sn-glycero-3-phosphocholine and that the availability of diradylglycerols and the turnover rate of ether-linked lipids would appear to be important factors in controlling the level (under 1%) of 1-alkyl-2-acyl-sn-glycero-3-phosphocholine in rat liver.