Phosphatidylcholine as a source of diacylglycerols in neuronal nuclei incubated in the presence of EGTA and CMP.

A neuronal nuclear fraction (N1), isolated from immature rabbit cerebral cortex, was preincubated with 1-[14C]palmitoyl-sn-glycero-3-phosphorylcholine and oleoyl-CoA. Most of the radioactivity was recovered in N1 phosphatidylcholine, and subsequent incubations in the presence of EGTA and CMP indicated an increase in radioactivity in N1 diacylglycerol and triacylglycerol which was matched by a decline in the labelling of N1 phosphatidylcholine. N1 phosphatidylcholine was also prelabelled using [14C]oleate and 1-acyl-sn-glycero-3-phosphorylcholine in vitro, or by intrathecal injection of [3H]oleate prior to N1 isolation. In the following incubations with EGTA and CMP there was a good correspondence between the radioactive decline in N1 phosphatidylcholine and the increase in radioactivity in N1 diacylglycerol. In all these experiments the generation of radioactive diacylglycerol depended upon the presence of EGTA and CMP in the incubations and could be largely inhibited by the addition of CDP-choline. During the prelabelling procedures noted above, other complex lipids had less of the total radioactivity than phosphatidylcholine and showed little or no decline in radioactivity in the presence of EGTA and CMP. In N1 preincubations with [14C]oleate and lysophosphatidylethanolamine, phosphatidylethanolamine could be more highly labelled than phosphatidylcholine, but in subsequent incubations with EGTA and CMP no decline was seen in phosphatidylethanolamine radioactivity. It is concluded that the back reaction of cholinephosphotransferase in N1 represents an active route for the production of diacylglycerols bearing palmitate and/or oleate.