Evidence that only newly made phosphatidylethanolamine is methylated to phosphatidylcholine and that phosphatidylethanolamine is not significantly deacylated-reacylated in rat hepatocytes.

The metabolism of the molecular species of phosphatidylethanolamine derived from [3H]ethanolamine and molecular species of phosphatidylcholine derived from [3H]ethanolamine or [methyl-3H]choline has been studied in rat hepatocytes. After an initial pulse of radioactivity for 1 h and a chase for up to 24 h, the cells were harvested and the incorporation of label into the various molecular species of phosphatidylethanolamine and phosphatidylcholine was determined. The incorporation and metabolism of choline- and ethanolamine-labeled phosphatidylcholine was consistent with deacylation of some species of phosphatidylcholine and reacylation to form molecular species of phosphatidylcholine with different fatty acyl components. In contrast, such remodeling of ethanolamine-labeled phosphatidylethanolamine was not evident. Radioactivity disappeared from all molecular species of phosphatidylethanolamine without an increase in any of the species of phosphatidylethanolamine. This radioactivity was recovered in water-soluble metabolites in the cells and medium. Phosphatidylethanolamine (16:0-22:6) had an initial turnover rate (5.8 nmol/h) which was two or more times that of any of the other major molecular species of phosphatidylethanolamine. The molecular species of phosphatidylethanolamine displayed biphasic turnover profiles. The second rate of decay of radioactivity between 12 and 24 h was 2-4 times slower than the initial decay rate. During the first 2 h of the chase period, phosphatidylcholine was a major metabolite of labeled phosphatidylethanolamine. Subsequently, there was minimal conversion of phosphatidylethanolamine to phosphatidylcholine which suggests that only newly made phosphatidylethanolamine is available as a substrate for methylation to phosphatidylcholine.