Metabolism of Glycerolipids

Phosphatidylethanolamine or phosphatidylcholine may be synthesized in mammalian tissues from diglycerides and cytidine diphosphate ethanolamine or cytidine diphosphate choline, respectively (1). Hydrolysis of the naturally occurring phosphatides has indicated that often the fatty acids at the @ position are unsaturated whereas those at (Y’ are more often saturated (2-4). Although it is possible that in vivo the CDP-ethanolamine diglyceride transferase is specific for those diglycerides with an o!-unsaturated-@-saturated configuration, there is as yet little evidence (5) to support this. Previous work in this laboratory has shown that phosphatidylcholine (diacylglycerophosphorylcholine) may be synthesized from acylglycerophosphorylcholine and acyl coenzyme A in an acyl transfer catalyzed by rat liver microsomes (6). Further work showed that there was a positional specificity in this transfer so that linoleic acid was preferentially incorporated into the p position whereas stearic acid went preferentially into the cr’ position (7). Although the fatty acid composition of diacylglycerophosphorylethanolamine could be derived from diacyl-GPCi by diglyceride interchange, we have now established that diacyl-GPE can also be formed directly from acyl-GPE. This latter reaction, analogous to that found for diacyl-GPC (7), allows selective formation of the fatty acid esters in diacyl-GPE without involving other lipid esters, e.g. diglycerides, as intermediates. Furthermore, the results indicate that in vitro the acyl transfer system has a selectivity similar to that which must be operating in z&o to esterify saturated acids at the (II’ position, and unsaturated acids at the @ position, of the phosphatidylethanolamines.