HEPATIC MONOACYLGLYCEROL ACYLTRANSFERASE IS REGULATED BY SN-1,2-DIACYLGLYCEROL AND BY SPECIFIC LIPIDS IN TRITON X-100 PHOSPHOLIPID-MIXED MICELLES

The lipid cofactor requirement of hepatic monoacylglycerol acyltransfe rase (MGAT) (EC 2.3.1.22) was studied in Triton X-100/lipid-mixed mice lles. Anionic phospholipids and anionic lysophospholipids stimulated M GAT activity, whereas fatty acids and sphingosine inhibited enzyme act ivity. Phosphatidic acid was a potent activator, stimulating MGAT 11-f old at 4.2 mol %. Kinetic studies revealed that phosphatidic acid, wit h an apparent K-alpha of 0.26 mol %, was a better activator than phos- phatidylserine, phosphatidylinositol, or cardiolipin. Of the anionic lysophospholipids, lysophosphatidic acid was a better activator than l ysophosphatidylserine, stimulating maximally at less than 3 mol %. Ole ate was a more potent inhibitor (K-i, 2.4 mol %) than sphingosine (K-i , 18.3 mol %). The dependence of MGAT on sn-2-monoacylglycerol was not cooperative in the absence or presence of anionic phospholipids, olei c acid, or sphingosine. The apparent K-m for sn-2-monoC18:1-glycerol w as 1.24 mol % in the presence of maximally activating phospholipid and 0.19 mol % when phospholipid was omitted. MGAT's product sn-1,2-diacy lglycerol was a weaker activator than the anionic phospholipids, but t he effects of diacylglycerol and phospholipid were additive. Activatio n by sn-1,2-diC18:1-glycerol was highly cooperative with a Hill coeffi cient of 3.6. Activation was specific for the sn-1,2-stereoisomer; nei ther 1,3-diacylglycerol nor the ether analogs of sn-1,2- or 1,3-diacyl glycerol were activators. Since several of the lipid modulators of MGA T activity are intracellular second messengers, these data suggest the possibility that regulatory links exist between signal transduction a nd the synthesis of complex lipids via the monoacylglycerol pathway.