The CMP-stimulated production of diacylglycerol and CDPdiacylglycerol in neuronal nuclei labelled with radioactive arachidonate.

A neuronal nuclear fraction (N1), isolated from immature rabbit cerebral cortex, was preincubated with [3H]arachidonate, ATP, CoA, Mg2+ and 1-acyl-sn-glycero-3-phosphocholine or 1-acyl-sn-glycero-3-phosphoinositol. Using the former lysophospholipid, a sizeable incorporation of radioactivity was seen in N1 phosphatidylcholine. In subsequent incubations in the presence of CMP and EGTA, there was a generation of radioactive diacylglycerol in N1 and a corresponding decline in phosphatidylcholine radioactivity. Both these changes could be blocked by the addition of CDPcholine. In incubations using N1 phosphatidylinositol or phosphatidylethanolamine prelabelled with [3H]arachidonate, no evidence was found to support a direct generation of diacylglycerol from these phospholipids. The back reaction of cholinephosphotransferase in N1 is likely the principal source of diacylglycerols bearing arachidonate. Using either lysophospholipid in the preincubations described in the opening sentence, more than half of the incorporated radioactivity derived from [3H]arachidonate was found in N1 phosphatidylinositol. In subsequent incubations with EGTA and CMP there was a production of radioactive CDPdiacylglycerol and a decline in radioactive phosphatidylinositol. Both events could be blocked by the presence of myo-inositol. Radioactive CDPdiacylglycerol, produced in N1 in the presence of CMP and EGTA, was converted back into phosphatidylinositol by the addition of myo-inositol. The production of CDPdiacylglycerol is likely the result of the back reaction of CDPdiacylglycerol:inositol phosphatidate transferase in N1.