Studies on the biosynthesis of pulmonary surfactant. The role of the methylation pathway of phosphatidylcholine biosynthesis in primate and non-primate lung.

The relative importance of the choline incorporation and methylation pathways of phosphatidylcholine biosynthesis was studied in the rat, rabbit, rhesus monkey and human lung. In vitro studies showed that phosphatidylethanolamine methyltransferase activity in the lungs of all species was only a fraction of that in rat liver. In vivo experiments were carried out to study the incorporation of an intravenously administered equimolar mixture of L-[methyl-14C]-methionine and [methyl-3H] choline into lipid inthe lung and liver of the rabbit and monkey. Over 90% of the total radioactivity incorporated into lipid was found in phosphatidylcholine. In the liver, methionine incorporation was 40--71% of that of choline in the rabbit and 99--120% in the monkey while in the lungs of both species it was less than 3%. On a weight basis, choline incorporation in the lung was 2--4 times that in the liver. These findings suggest that the methylation pathway is of little quantitative significance in the biosynthesis of phosphatidylcholine in primate and non-primate lung.