DIRADYLGLYCEROLS ALTER FATTY-ACID INHIBITION OF MONOACYLGLYCEROL ACYLTRANSFERASE ACTIVITY IN TRITON X-100 MIXED MICELLES

The activity of hepatic monoacylglycerol acyltransferase (MGAT) (EC 2. 3.1.22), a developmentally expressed microsomal enzyme, is inhibited b y long-chain fatty acids, and stimulated by its product 1,2-diacyl-sn- glycerol. Because the quantities of fatty acids and diacylglycerols ar e likely to vary in membranes during different physiological condition s and could thereby alter MGAT activity, we examined their combined ef fects on MGAT in Triton X-100/phospholipid mixed micelles, MGBT's prod uct, 1,2-diC18:1-sn-glycerol, which is also normally a cooperative act ivator of the activity, reversed the 50% inhibition caused by 10 mol % oleic acid. The presence of oleic acid also allowed low concentration s (<10 mol %) of 1,2-diC18: 1-sn-glycerol to stimulate MGAT activity w ithout the lag that is observed in the absence of fatty acid. At 12.6 mol %, 1,2-monoC18:1-sn-glycerol ether, which alone has no effect on M GAT activity, became an activator in the presence of 10 mol % oleic ac id. Kinetic studies revealed that in the presence of 15 mol % oleic ac id, 1,2-diC18:1-sn-glycerol ether increased the apparent V-max by 3.8- fold while minimally altering the apparent K-m for palmitoyl-CoA. Othe r neutral lipids including triC18:1-glycerol, ceramide, and cholestero l oleate did not stimulate MGAT in either the presence or the absence of fatty acid. Assay conditions altered MGAT's apparent relative prefe rences for potential monoradylglycerol substrates. The presence of pho spholipids and of MGAT's 1,2-diacyl-sn-glycerol product increased the enzyme's apparent preference for its 2-monoacyl-sn-glycerol substrate by selectively increasing the apparent V-max 2.7-fold only when 2-mono C18:1-sn-glycerol was the substrate. Thus, in addition to previously r eported regulation of MGAT by phospholipids and intracellular lipid se cond messengers, these studies lend additional support to the hypothes is that changes in other membrane-associated lipids, such as long-chai n fatty acids and diradylglycerols, may also profoundly alter the acti vity of MGAT.