Phosphatidylcholine produced in rat synaptosomes by N-methylation is enriched in polyunsaturated fatty acids.

Abstract

Rat brain synaptosomes contain an enzyme, phosphatidylethanolamine N-methyltransferase (EC 2.1.1.17), that catalyzes the methylation of phosphatidylethanolamine to form its mono-, di-, and trimethyl (phosphatidylcholine) derivatives. Synaptosomal phosphatidylethanolamine is much richer in polyunsaturated fatty acids (43.4%) than is synaptosomal phosphatidylcholine (4.6%). It thus seemed possible that the phosphatidylcholine derived via the Nmethylation of phosphatidylethanolamine might also be especially enriched in polyunsaturated fatty acids. To test this hypothesis, we examined the incorporation of PH]methyl groups into various molecular species of phosphatidylcholine, by incubating rat synaptosomes for 10, 30, or 90 min in a medium containing S-adenosyl[methyl-3H]methionine.Phosphatidylcholine was extracted and separated from other lipids by TLC, after which its molecular species were isolated by argentation TLC (which distinguishes among the phospholipid molecules by the degree of unsaturation of their fatty acid moieties.) We found that approximately 65% of the [3H]methyl incorporated into phosphatidylcholine during the incubation period was present in the fraction associated with pentaene or hexaene fatty acids; an additional 30% was present in the tetraene fraction, while the remaining phosphatidylcholine radioactivity was distributed between the dienes and monoenes. Similar distributions were observed among synaptosomes incubated for 10 or 30 min; however, after 90 min the phosphatidyl[3Hjcholine contained proportionately less of the tetraenes. These observations indicate that neuronal phosphatidylcholine molecules formed via N-methylation are especially richer in polyunsaturated fatty acids, and they raise the possibility that these molecules constitute a distinct pool with particular physiologic functions.Three synthetic pathways can produce phosphatidylcholine (PtdCho) molecules in cells: the Kennedy pathway, which combines CDP-choline with 1,2-diacylglycerol; the baseexchange pathway, which exchanges free choline with a serine or ethanolamine in phosphatidylserine or phosphatidylethanolamine (PtdEtn); and the sequential N-methylation of PtdEtn, forming its mono-, di-, and trimethyl (PtdCho) derivatives. The latter is the only pathway capable of producing new choline moieties in the brain, which otherwise depends on the circulation for its choline supply (1, 2). This pathway, catalyzed by phosphatidylethanolamine N-methyltransferase (PtdEtnMeTase; EC 2.1.1.17), has been demnonstrated in brain (3, 4), its activity being highest in synaptosomes (5).The fatty acid composition of synaptosomal PtdEtn, which is the precursor for the PtdCho that is formed via methylation (and a precursor for that formed via the base exchange pathway), differs markedly from that of PtdCho, containing 43.4% polyunsaturated fatty acids as opposed to 4.6% in PtdCho. Two hypotheses have been proposed (6)

Light Activation of Phosphatidylethanolamine N-Methyltransferase in Rod Outer Segments and its Modulation by Association States of Transducin

“…Bovine ROS do not support the de novo synthesis of phosphatidylcholine (PC), but they contain an alternative pathway catalyzed by PE N-MTase (Roque and Giusto, 1995). The PE N-MTase activity in ROS could be responsible for the synthesis of PC molecules enriched in polyunsaturated fatty acids, a phenomenon which is in agreement with previous observations in the brain and liver (Ridgway and Vance, 1988 ;Tacconi and Wurtman, 1985). It has recently been demonstrated that PE N-MTase in isolated ROS is modulated by light and by a mechanism involving specific protein kinases and phosphatases in vitro .…”

Administration of Myo-inositol Plus Ethanolamine Elevates Phosphatidylethanolamine Plasmalogen in the Rat Cerebellum

“…In our studies of rat brain, we were unable to detect any 13 C labeling on PtdC in either whole brain experiments (12) or in the current investigation involving specific brain areas. Furthermore, the literature suggests that the activity of PEMT is extremely low in adult brain tissue thus making any contributions from the pathway negligible (28)(29)(30). We have also conducted NMR studies in neonate rat brains following administration of 13 C-labeled ethanolamine, which also failed to show any appreciable synthesis of PtdC via the PEMT pathway (unpublished data).…”

Phospholipase A2 and phospholipase C are activated by distinct GTP-binding proteins in response to alpha 1-adrenergic stimulation in FRTL5 thyroid cells.

“…In those reports the identity of the phospholipases mediating arachidonic acid release was not assessed directly. The Phosphatidylcholine arising from the receptor-mediated methylation of phosphatidylethanolamine is rich in arachidonic acid (28,29). In unstimulated FRTL5 cells, arachidonic acid is preferentially incorporated into phosphatidylethanolamine (R.M.B., unpublished data).…”