Acyl specificity of CDPcholine:l,2-diacylglycerol cholinephosphotransferase in rat lung

Abstract

Under optimal conditions, rat lung microsomal cholinephosphotransferase (EC 2.7.8.2) activity is markedly stimulated by exogenously added 1,2-sn-diacylglycerols containing an unsaturated fatty acid at the 2-position. Diacylglycerols containing long-chain saturated fatty acids at the 1- and 2-positions did not stimulate the incorporation of CDP[14C]choline above the incorporation observed with the endogenous diacylglycerols present in the microsomal preparation. 1-Oleoyl,2-palmitoyl-sn-glycerol was also ineffective in stimulating phosphatidylcholine synthesis. Diacylglycerols containing linoleate or linolenate at the 2-position were not as effective as 1-palmitoyl,2-oleoyl-sn-glycerol. Identical stimulations were observed with the latter diacylglycerol and 'natural' diacylglycerols prepared from egg-yolk phosphatidylcholine or pig liver phosphatidylcholine. Marked selectivities were observed with diacylglycerols containing two unsaturated fatty acids. Only minor amounts of 1,2-[14C]dipalmitoyl-sn-glycerol were incorporated into phosphatidylcholine, even when this radioactive diacylglycerol was dispersed with 'egg' diacylglycerols. When CDP[14C]choline was incorporated into rat lung microsomal lipids with endogenous diacylglycerols or diacylglycerols endogenously generated by phospholipase C (EC 3.1.4.3) (Bacillus cereus), little radioactivity was associated with the disaturated species of phosphatidylcholine.It has previously been suggested that cholinephosphotransferase catalyses the rate-limiting reaction in the biosynthesis of phosphatidylcholine by lung and that this enzyme is specifically induced in fetal lung by glucocorticoids. The present results indicate that these proposals are untenable and must be reevaluated. These experiments also suggest that dipalmitoyl phosphatidylcholine is not synthesized readily by the de novo pathway for lecithin synthesis and must be produced through reacylation or transesterification mechanisms.