The formation of phosphatidylethanolamine from diglyceride by extracts of Escherichia coli

Extensively dialyzed cell-free homogenates or washed particulate fractions, of Escherichia coli in the presence of added CTP, Mg2+, serine, and rac-glycerol 3-phosphate, incorporated [32P]phosphatidic acid into phosphatidylglycerol and cardiolipin but not into phosphatidylethanolamine. [14C]Phosphatidic acid could be incorporated by these preparations into phosphatidylethanolamine in the absence of added CTP and Mg2+. CDP-diglyceride did not significantly affect the formation of [14C]phosphatidylethanolamine from [14C]phosphatidic acid. 14C-labeled diglyceride was also readily incorporated into phosphatidylethanolamine in the absence of added cofactors by both homogenate and particulate fraction. Serine stimulated the incorporation somewhat whereas CTP + Mg2+ diminished this conversion slightly because of concurrent [14C]phosphatidic acid and [14C]phosphatidylglycerol formation. CDP-diglyceride did not significantly affect the conversion of 14C-labeled diglyceride to phosphatidylethanolamine. Dialyzed 14C-labeled cytosol fractions of E. coli, obtained from cells grown in medium containing [14C]serine, transferred some of their label to phosphatidylethanolamine in the presence of fresh particulate fraction. This transfer was stimulated by added diglyceride. The results indicate that phosphatidylethanolamine can be synthesized from exogenous diglyceride by a pathway which does not require CDP-diglyceride as an intermediate but which likely makes use of an endogenous cofactor supplying phosphorylethanolamine and (or) phosphorylserine.