Metabolic pathways for diacylglycerol biosynthesis and release in the midgut of larval Manduca sexta

The pathway for the synthesis of diacylglycerol in larval Manduca sexta mid gut was studied. Fifth instar larvae were fed with [9,10-H-3]-oleic acid-la beled triolein and the incorporation of the label into lipid intermediates was analyzed as a function of time. The results showed that the triacylglyc erol was hydrolyzed to fatty acids and glycerol in the midgut lumen. In mid gut tissue, the labeled fatty acids were rapidly incorporated into phosphat idic acid, diacylglycerol and triacylglycerol, but no significant labeling of monoacylglycerol was observed. Dual-labeling experiments were performed in order to characterize the kinetics of diacylglycerol biosynthesis in the midgut, its incorporation into hemolymph lipophorin and its clearance from hemolymph. The results Even best described by a model in which the rate-li miting step in diacylglycerol biosynthesis was the uptake of fatty acid fro m the lumen of the midgut. Once in the cell the fatty acid was rapidly inco rporated in phosphatidic acid and diacylglycerol. Diacylglycerol was conver ted to triacylglycerol or exported into hemolymph. The interconversion of d iacylglycerol and triacylglycerol was fairly rapid, suggesting that triacyl glycerol serves as a reservoir from which diacylglycerol can be produced. T his mechanism permits the cell to maintain a low steady-state concentration of diacylglycerol and yet efficiently absorb fatty acids from the lumen of the midgut. (C) 2000 Elsevier Science Ltd. All rights reserved.