M.C. Van Heusden

THE RECYCLING OF PROTEIN COMPONENTS OF THE FLIGHT-SPECIFIC LIPOPHORIN IN LOCUSTA MIGRATORIA

Abstract The metabolism of locust lipophorin A + during lipid delivery to the flight muscle and lipid loading at the fat body was studied in vitro . Protein C 2 was shown to be released upon hydrolysis of lipophorin A + -carried diacylglycerol by the flight muscle lipoprotein lipase. This in vitro released protein C 2 was shown to reassociate with lipophorin A y upon hormone-induced lipid mobilization from fat body in vitro . These results demonstrate the reversibility of the association of protein C 2 with lipophorin A y and support the shuttle function of the protein components of locust lipophorin A + in lipid transport.

In vitro studies on hormone-stimulated lipid mobilization from fat body and interconversion of haemolymph lipoproteins of Locusta migratoria

Both adipokinetic hormone and octopamine have a stimulating effect on lipid release from locust fat body in vitro, when incubated in diluted haemolymph. The presence of adipokinetic hormone results in the formation of the flight-specific haemolymph lipoprotein A+ accepting the increased amount of lipids released into the incubation medium. In contrast, interconversions of lipoproteins do not occur when octopamine is added to the incubation medium, which is in line with the expectations: the lipid-mobilizing effect of octopamine is a limited and short-term effect. When fat body tissue is incubated with isolated haemolymph protein fractions, the lipid-mobilizing effect of adipokinetic hormone only occurs when the incubation medium contains both lipoprotein, Ay and protein fraction C, resulting in the formation of lipoprotein A+. In similar control incubations with the hormone omitted, some lipoprotein A+ is also formed (concomitant with a slight amount of lipid released), though significantly less than in incubations with hormone. Besides a stimulating function on lipolytic processes in the fat body, adipokinetic hormone is suggested to influence haemolymph lipoprotein rearrangement. A possible counteracting function of another factor in the haemolymph is discussed.

Lipoprotein lipase activity in the flight muscle of Locusta migratoria and its specificity for haemolymph lipoproteins

Abstract Lipoprotein lipase activity in flight muscle homogenates of Locusta migratoria was measured, using natural radiolabelled lipoproteins as substrates. The flight specific lipoprotein A + (or low density lipophorin) stimulated lipoprotein lipase activity several-fold compared to the resting lipoprotein A y (or high density lipophorin). However, with the high mol. wt lipoprotein fraction O AKH as a substrate, lipase activity was even doubled compared to lipoprotein A + . Lipase activity was not increased in flight muscle homogenates of insects which had flown. Neither adipokinetic hormone, nor octopamine had any direct effect on lipoprotein lipase activity. Aspects of hormonal regulation and apoprotein activation of the locust flight muscle lipoprotein lipase are discussed and compared with the model for vertebrate lipoprotein lipase.

Partial purification of locust flight muscle lipoprotein lipase (LpL): apparent differences from mammalian LpL

1. An attempt was made to purify lipoprotein lipase (LpL) from the flight muscle of the migratory locust based on affinity for heparin, which is known to avidly bind mammalian LpL. 2. However, locust LpL appeared to completely lack this property, which indicates that the suggested membrane-binding of locust LpL is very different from that of mammalian LpL: a heparin-like glycosaminoglycan is not involved. 3. Since locust LpL lacks heparin affinity, other purification methods were assayed. Solubilization of locust LpL was obtained by the detergent Tween 20. 4. Though both anion and cation exchange chromatography resulted in the complete loss of enzyme activity, partial purification of locust LpL was achieved by gel filtration chromatography.