Abstract
Diacylglycerol was generated in vitro in rat lung microsomes by forming phosphatidic acid via sn-glycerol-3-phosphate acyltransferase followed by the hydrolysis of the phosphatidic acid by phosphatidate phosphohydrolase. Diacylglycerol concentrations of 35 to 50 nmol/mg of microsomal protein were obtained. Cholinephosphotransferase activity was determined in microsomes by measuring the conversion of endogenously generated [14C]diacylglycerol to phosphatidylcholine. Reaction rates of 14 to 16 nmol/min/mg of protein were obtained with a 30-s reaction. Diacylglycerol which was primarily dipalmitoylglycerol was produced when palmitic acid was used in the sn-glycerol-3-phosphate acyltransferase reactions. Dipalmitoylphosphatidylcholine was formed via cholinephosphotransferase from the dipalmitoylglycerol with an apparent maximal velocity of 20 nmol/min/mg of protein. When oleic acid was used instead of palmitic acid, the apparent maximal velocity for cholinephosphotransferase was 26 nmol/min/mg of protein. The apparent Km values for the two different diacylglycerol substrates were the same (28.5 nmol/mg of protein). Diacylglycerols, with different molecular species composition, were generated using a variety of fatty acids and fatty acid mixtures. The phosphatidylcholine formed from these diacylglycerols had the same molecular species profiles as the diacylglycerol used as the substrate. The relative reaction rates with the different diacylglycerols were essentially the same except when 20:4 and 22:6 fatty acids were used individually, in which case the rates were lower. We conclude that cholinephosphotransferase readily forms dipalmitoylphosphatidylcholine from endogenously generated dipalmitoylglycerol and that the cholinephosphotransferase reaction is generally nonselective for the diacylglycerol substrate.
Highlights
Diacylglycerolwas generated in vitro in rat lung way, which is the major route for the synthesis of phosphatimicrosomes by forming phosphatidic acidvia sn-glyc- dylcholine by de nouo synthesis via cholinephosphotransfererol-3-phosphate acyltransferase followed by the hydrolysis of the phosphatidicacidbyphosphatidate phosphohydrolase
Cholinephosphotransferaseactivity was determined in microsomes by measuring the conversion of endogenously generated [‘4C]diacylglycerol to phosphatidylcholine.Reaction rates of 14 t o 16 nmol/min/mg of protein were obtainedwith a 30-s reaction
We recently reported results which suggested that dipalmitoylphosphatidylcholine can be synthesized readily by the de nouo CDP-choline pathway and that theremodeling pathparent maximal velocity o2f0 nmol/min/mg of protein. way need not be the exclusive route for the synthesis of
Summary
I N VITRO FORMATION OF DIPALMITOYLPHOSPHATIDYLCHOLINE AND GENERAL LACK OF SELECTIVITY USING ENDOGENOUSLY GENERATED DIACYLGLYCEROL*. Results from radioactive tracer experiments [4,5,6,7,8,9,10,11] and from studies on the diacylglycerolspecificities of cholinephosphotransferase [12,13,14,15] have been interpreted to indicate that dipalmitoylphosphatidylcholine is synthesized by remodeling of unsaturated species of phosphatidylcholine by the combined action of phospholipase Az (EC 3.1.1.4) and lysolecithin acyltransferase (EC 2.3.1.23).
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