Some studies on the biosynthesis of the molecular species of phosphatidylcholine from rat lung and phosphatidylcholine and phosphatidylethanolamine from rat liver

1. 1. At different time intervals after injection of [I(3)-3H]glycerol, the incorporation of glycerol into the various molecular species of phosphatidylcholine and phosphatidylethanolamine from rat liver, and phosphatidylcholine from rat lung was determined. 2. 2. The results indicate that, in liver, a de novo synthesis is primarily operating in the biosynthesis of linoleic acid-containing molecules of lecithin and of the hexaenoic molecular species of phosphatidylethanolamine. An acylation of monoacyl derivatives of these phospholipids is suggested to play an important role particularly in the formation of arachidonic acid containing molecular species of these phospholipids. 3. 3. In lung, the de novo synthesis was found to contribute also primarily to the linoleic acid-containing lecithins, though it also represents an important pathway for the synthesis of tetraenoic, monoenoic and, perhaps to a lesser extent, disaturated lecithins. A deacylation-reacylation mechanism may contribute significantly to the formation of dipalmitoyl lecithin, a major constituent of lung pulmonary surfactant. 4. 4. Acylation of I-palmitoyl-sn-glycero-3-phosphorylcholine with various labeled fatty acids or acyl-CoA esters was studied in the presence of microsomes from rat lung and liver. In the presence of microsomes from lung a significant uptake of palmitic acid was observed into the 2-position of lecithin, this in strong contrast to the findings with liver microsomes where only a very limited uptake of palmitic acid was observed. The results endorse the findings from the in vivo studies that the acylation of monoacyl-sn-glycero-3-phosphorylcholine may play an important additional role in maintaining the high level of dipalmitoyl lecithin in lung. 5. 5. Comparison of the composition of phosphatidylethanolamine and lecithin from lung suggested, in support of previous observations by other investigators, that the methylation of phosphatidylethanolamine does not represent an important pathway for the formation of dipalmitoyl lecithin