METABOLIC-FATE AND TURNOVER RATE OF HEMOLYMPH FREE FATTY-ACIDS IN ADULT MANDUCA-SEXTA

The transport, concentration, turnover rate and metabolic fate of free fatty acids (FFA) in adult Manduca sexta were analyzed. FFA are trans ported exclusively by lipophorin, in either of its two forms, HDLp or LDLp, and no evidence was found for a FFA-protein complex similar to t he vertebrate FFA-albumin complex. The half-life of hemolymph FFA is a bout 2 min. In whole adult insects most of the hemolymph FFA (75%) are (re)esterified into phospholipid (PL), diacylglycerol (DG), and triac ylglycerol (TG), while the remaining 25% is oxidized to CO2 with a sma ll amount (2.5%) converted to ketones. In the hemolymph of adults, the major product derived from FFA was DG. Resting M. sexta adults oxidiz e about 0.35 mg of FFA/h. During the initial stages of flight, the rat e of hemolymph FFA utilization does not increase significantly. In ord er 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 i n 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 amoun t 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 l ipophorin lipase in flight muscle. Based on these studies, it is concl uded that, in resting adult M. sexta, hemolymph-FFA represent an impor tant route for distribution of fatty acids among tissues. However, it remains to be determined which tissues, in addition to fat body, utili ze hemolymph-FFA.