ABSTRACT
Phosphatidylcholine (PC), the major phospholipid of mammalian cells, is synthesized primarily by the cytidine diphosphate (CDP)-choline pathway. There is a second route for PC synthesis which involves N-methylation of phosphatidylethanolamine. Saturates, monoenes, dienes, and trienes are the major molecular species of PC formed by the CDP-choline pathway. However, as discussed in a previous section, PC formed via methylation of PE is rich in l-palmitoyl-2-docosahexaenoyl-PC. The relative lack of input into lipoprotein PC by phosphatidylethanolamine (PE) methylation was further demonstrated by treating hepatocytes with deazaadenosine. Analysis of PE N-methyltransferase molecular specificity in rat brain revealed polyunsatu-rated-rich species of PC to be the major methylation products. In vitro labeling of synaptosomal PC and in vivo labeling of whole rat brain PC showed tetra-, penta-, and hexaenoic species of PC to be the primary products of methylation. The activation by PC, and perhaps by other lipids, seems to be related to a requirement for a full boundary layer of phospholipid.
