Metabolic fate and turnover rate of hemolymph free fatty acids in adult Manduca sexta

Abstract

The transport, concentration, turnover rate and metabolic fate of free fatty acids (FFA) in adult Manduca sexta were analyzed. FFA are transported exclusively by lipophorin, in either of its two forms, HDLp or LDLp, and no evidence was found for a FFA-protein complex similar to the vertebrate FFA-albumin complex. The half-life of hemolymph FFA is about 2 min. In whole adult insects most of the hemolymph FFA (75%) are (re)esterified into phospholipid (PL), diacylglycerol (DG), and triacylglycerol (TG), while the remaining 25% is oxidized to CO 2 with a small amount (2.5%) converted to ketones. In the hemolymph of adults, the major product derived from FFA was DG. Resting M. sexta adults oxidize about 0.35 mg of FFA/h. During the initial stages of flight, the rate of hemolymph FFA utilization does not increase significantly. In order to account for the high rates of lipid utilization in flying adults, approx. 8 mg/h [Ziegler and Schulz (1986) J. Insect Physiol. 32, 903–908], there must be a dramatic increase in DG utilization. We suggest that fatty acids are transported from the fat body to flight muscle in the form of DG, instead of FFA, because the very rapid metabolism of FFA in all tissues, but especially in fat body, would limit the amount of fatty acid reaching flight muscle; whereas with DG, the delivery of fatty acids can be targeted to flight muscle by the presence of a lipophorin lipase in flight muscle. Based on these studies, it is concluded that, in resting adult M. sexta, hemolymph-FFA represent an important route for distribution of fatty acids among tissues. However, it remains to be determined which tissues, in addition to fat body, utilize hemolymph-FFA.