Lipoproteins serve a vital function as transport vehicles of lipophilic biomolecules. Lipoprotein metabolism is affected by the presence of specific apoproteins and the action of lipid transfer proteins. We will employ an insect model system to study plasma lipoprotein metabolism. We plan to continue our investigation of the major hemolymph lipoprotein, lipophorin, and the role of a lipid transfer protein and a specific apolipoprotein in its metabolism. The physical properties of Manduca sexta lipid transfer particle (LTP), which catalyzes net transfer of lipid mass between isolated lipoproteins in vitro, will be determined. Characterization of the apoprotein and lipid components of this very high density lipoprotein will be performed. The requirement of individual apoproteins and the lipid component of LTP for catalysis of lipid transfer will be determined. Antibodies directed against LTP will be used to investigate its role in the mechanism of hormone-induced net uptake and non-endocytotic delivery of lipoprotein-associated lipids to tissues in vitro and in vivo. In addition, the role of LTP in mediating lipid transfer processes involved in uptake and delivery of ingested lipids during larval life will be determined. LTP will also be used as a tool to modify the lipid content of lipoproteins in order to investigate the effect of particle lipid content on apolipophorin III binding to lipophorin. The ability of free apolipophorin III to exchange with lipoprotein- bound apolipophorin III will be assessed. These studies should greatly increase our understanding of lipoprotein metabolism in insects and also provide basic information regarding pathways of lipoprotein metabolism which involve non-endocytotic transfer of lipoprotein-associated lipids to cells and from cells to lipoprotein.
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