Choline metabolism in placenta: evidence for the biosynthesis of phosphatidylcholine in microsomes via the methylation pathway.

Microsomes from human, mouse and rat placenta were found to contain enzymatic activity which methylates the phospholipids phosphatidylethanolamine (PE), phosphatidyl-N-monomethylethanolamine (PMME) and phosphatidyl-N,N-dimethylethanolamine (PDME) to form phosphatidylcholine (PCh) with 3H-methyl-S-adenosyl-l-methionine as the methyl donor. The three labelled reaction products were isolated by solvent extraction and separated on thin-layer chromatography (TLC) plates. The endogenous methyltransferase activity was low, indicating that the methylation pathway is quantitatively not important for the synthesis of free choline to meet the fetal needs. The distribution of 3H-methyl among PMME, PDME and PCh revealed fairly even labelling of all products when analysed by TLC. Addition of authentic PE, PMME and PDME to a level of approximately 2.5 mM stimulated the incorporation of 3H-methyl into the total lipid-soluble fraction with all three substrates, but was most pronounced with PMME. Present observations suggest that all three methylation steps were catalysed by one enzyme with a pH optimum of 9.0 in a reaction that does not require Mg++.