ROLE OF FATTY-ACID-BINDING PROTEIN IN LIPID-METABOLISM OF INSECT FLIGHT-MUSCLE

Since insect flight muscles are among the most active muscles in natur e, their extremely high rates of fuel supply and oxidation pose intere sting physiological problems. Long-distance flights of species like lo custs and hawkmoths are fueled through fatty acid oxidation. The lipid substrate is transported as diacylglycerol in the blood, employing a unique and efficient lipoprotein shuttle system. Following diacylglyce rol hydrolysis by a flight muscle lipoprotein lipase, the liberated fa tty acids are ultimately oxidized in the mitochondria. Locust flight m uscle cytoplasm contains an abundant fatty acid-binding protein (FABP) . The flight muscle FABP of Locusta migratoria is a 15 kDa protein wit h an isoelectric point of 5.8, binding fatty acids in a 1:1 molar stoi chiometric ratio. Binding affinity of the FABP for long-chain fatty ac ids (apparent dissociation constant K(d) = 5.21 +/- 0.16 muM) is howev er markedly lower than that of mammalian FABPs. The NH2-terminal amino acid sequence shares structural homologies with two insect FABPs rece ntly purified from hawkmoth midgut, as well as with mammalian FABPs. I n contrast to all other isolated FABPs, the NH2 terminus of locust fli ght muscle FABP appeared not to be acetylated. During development of t he insect, a marked increase in fatty acid binding capacity of flight muscle homogenate was measured, along with similar increases in both f atty acid oxidation capacity and citrate synthase activity. Although c onsiderable circumstantial evidence would support a function of locust flight muscle FABP in intracellular uptake and transport of fatty aci ds, the finding of another extremely well-flying migratory insect, the hawkmoth Acherontia atropos, which employs the same lipoprotein shutt le system, however contains relatively very low amounts of FABP in its flight muscles, renders the proposed function of FABP in insect fligh t muscles questionable.