Insect hemolymph lipophorin: a mechanism of lipid transport in insects.

Abstract

Lipophorin, formerly called the "diacylglycerol-carrying lipoprotein," exists in the hemolymph of many insects including locust, cockroach, and silkworm. A rapid and efficient method has been developed for the purification of lipophorin, which includes a specific precipitation under low ionic concentration and DEAE-cellulose column chromatography. The final preparation of lipophorin is highly homogeneous, as judged by gel electrophoresis, electronmicroscopy, and immunodiffusion. Molecules of lipophorin from the above three insects are almost globular in shape with a diameter of 13-16 nm. Molecular weights are 600,000-700,000, and the lipid content totals 40-50%. The lipids are comprised of diacylglycerol, cholesterol, and phospholipid. Of particular interest is that the locust and cockroach lipophorin contains large amounts of hydrocarbons in addition to the above lipids. Apoprotein of lipophorin consists of two-non-identical subunits, heavy chain (M.W. 250,000) and light chain (M.W. 85,000); carbohydrate (mainly mannose) is covalently associated only with the heavy chain. Tests of the physiological function of insect lipophorin indicate that it has multiple roles as a true carrier and a reusable shuttle in transporting diacylglycerol, cholesterol, and hydrocarbon from sites of storage, absorption, and synthesis to sites where these lipids are utilized as metabolic fuel, precursors of triacylglycerol and phospholipid synthesis, or structural components of cell membrane and cuticle. In view of this functional multiplicity as a true carrier and reusable shuttle, therefore, the insect lipophorin is unique lipoprotein and essentially different from mammalian plasma lipoproteins.