PLASMENYLCHOLINE -O-ALK-1'-ENYL-2-ACYL-SN-GLYCERO-3-PHOSPHOCHOLINE) BIOSYNTHESIS IN GUINEA-PIG HEART AND LIVER - CHOLINEPHOSPHOTRANSFERASE IS A BIFUNCTIONAL ENZYME FOR THE SYNTHESIS OF PHOSPHATIDYLCHOLINE AND PLASMENYLCHOLINE

Plasmenylcholine is present in significant proportion (32 % of choline phosphoglycerides) in the guinea-pig heart but exists as a minor comp onent (3 % of choline phosphoglycerides) in the guinea-pig liver. In t his study, the biosynthesis of plasmenylcholine in these two organs wa s examined. The organs were perfused with labelled choline for 15 min and chased with unlabelled choline for up to 7 h. The labelling of pho sphatidylcholine was 6-fold higher than that of plasmenylcholine in th e heart and about 60-fold higher in the liver. However, the same label ling ratio was maintained throughout the chase period in both organs. Alterations in the specific radioactivity of CDP-choline caused corres ponding changes in the labelling of phosphatidylcholine and plasmenylc holine. Our results suggest that in guinea-pig heart and liver, CDP-ch oline is the immediate precursor of biosynthesis of phosphatidylcholin e and plasmenylcholine. The biochemical cause for the difference in th eir rates of formation between the two organs was explored. The enzyme activities for the formation of both choline phosphoglycerides were d etermined. The two reactions share the same characteristics, and 1,2-d iacylglycerol and 1-alk-1'-enyl-2-acylglcerol were found to be mutuall y inhibitory in a competitive fashion. The pool sizes of 1,2-diacylgly cerol and 1-alk-1'-enyl-2-acylglycerol were determined, and their rati os were found to be 42 in the heart and 422 in the liver. We conclude that cholinephosphotransferase catalyses the formation of both phospha tidylcholine and plasmenylcholine in the guinea-pig tissues and the ra te of plasmenylcholine biosynthesis is dependent on the availability o f 1-alk-1'-enyl-2-acylglycerol. Plasmenylcholine biosynthesis is also subjected to modulation by the 1,2-diacylglycerol content of the tissu e.