Selective retention of essential fatty acids: the role of hepatic monoacylglycerol acyltransferase

In the suckling rat, chick embryo, and hibernating marmot, fatty acids provide the major source of energy, and despite the high rate of hepatic beta-oxidation, these animals selectively retain long-chain polyunsaturated derivatives of C18:2n-6 and C18:3n-3. To determine whether the hepatic microsomal activity monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22) could provide a mechanism to selectively acylate monoacylglycerols that contain essential fatty acids, we tested the ability of MGAT activity from each of the three species to acylate sn-2-monoC18:1-, sn-2-monoC18:2-, sn-2-monoC18:3-, and sn-2-monoC20:4-glycerols. Hepatic MGAT activity acylated sn-2-monoC18:3-glycerol and sn-2-monoC18:2-glycerol in preference to sn-2-monoC18:1-glycerol in each of the three different lipolytic animals. MGAT's acyl group specificity could not be explained by invoking differences in membrane fluidity because the apparent affinity for sn-2-monoC20:4-glycerol was not increased. Further, sn-2-monoC18:3-glycerol remained a preferred substrate under assay conditions when both the C18:3 and C18:1 species were present in equal amounts. As would be predicted in the presence of high activity of a selective MGAT, the hepatic glycerolipids from neonatal rats showed increases in dienoic, trienoic, and C22:6 fatty acids and relative decreases in monoenoic, saturated, and C20:4 fatty acids. We hypothesize that, during lipolysis, the reacylation of sn-2-monoacylglycerols by MGAT may provide a mechanism by which essential fatty acids are retained within specific tissues.