Biosynthesis and secretion of insect lipoprotein.

Abstract

Biosynthesis of high density lipophorin (HDLp) was studied in larvae and adults of the migratory locust, Locusta migratoria. In an in vitro system, fat bodies were incubated in a medium containing a mixture of tritiated amino acids. Using SDS-PAGE and immunoblotting, it was shown that larval and adult fat bodies secreted both HDLp apoproteins, apolipophorin I (apoLp-I) and apolipophorin II (apoLp-II). Radiolabel was recovered in both apoproteins, indicative of de novo synthesis. The density of the fractions containing the apoproteins synthesized and secreted by larval and adult fat bodies was determined by density gradient ultracentrifugation. A radiolabeled protein fraction was found at density 1.12 g/ml. Using an enzyme-linked immunosorbent assay for detecting apoLp-I and apoLp-II, it was demonstrated that both apoproteins were present in this fraction, which had a density identical to that of circulating HDLp in hemolymph. Lipid analysis revealed that it contained phospholipid, diacylglycerol, sterol, and hydrocarbons. From these results it is concluded that the fat body of the locust synthesizes both apoLp-I and apoLp-II, which are combined with lipids to a lipoprotein particle that is released into the medium as HDLp.