BIOSYNTHETIC ROUTE OF LOCUST APOLIPOPHORIN-III ISOFORMS

Insect apolipophorin III (apoLp-III) plays a key role in the enhanced diacylglycerol transport during insect flight. For apoLp-III of the mi gratory locust, two different isoforms have been described (apoLp-IIIa and -b), displaying different N-termini and isoelectric points; each of the isoforms is however equally well capable to perform its functio n in lipid transport. In the present report the biosynthetic route of the apoLp-III isoforms is elucidated. Immunoprecipitation of media fro m in vitro fat body incubations and analysis by sodium dodecyl sulfate -polyacrylamide gel electrophoresis demonstrated that the locust fat b ody synthesized and secreted apoLp-III. ApoLp-III levels in the hemoly mph showed that in young adults, apoLp-III concentrations were only ve ry low (1-3 mg/ml). During adult maturation, however, the apoLp-III co ncentration increased rapidly to approximately 17 mg/ml. During apoLp- III elevation, the apoLp-IIIa:-b ratio remained equal or in the favour of the a-isoform, while in adults from approximately 12 days after ad ult ecdysis apoLp-IIIb was the most abundant isoform. Analysis of the protein by native polyacrylamide gel electrophoresis showed that only the apoLp-IIIa form was secreted. Injection of radiolabeled apoLp-IIIa into the hemolymph of adult locusts resulted in a slow conversion int o apoLp-IIIb.