The role of apolipophorin III in in vivo lipoprotein interconversions in adult Manduca sexta.
Sustained flight in the moth, Manduca sexta, necessitates lipid mobilization and transport to flight muscle, a process mediated by the adipokinetic hormone. An adult specific high density lipophorin (lipoprotein, HDLp-A, Mr = 7.68 X 10(5)) accepts diacylglycerol from the fat body, increasing in size and decreasing in density, to give a low density lipophorin (lipoprotein, LDLp, Mr = 1.56 X 10(6)). During this process, several molecules of the small apolipoprotein, apolipophorin III (apoLp-III), are added to the two molecules originally present in HDLp-A. A study of the time course of adipokinetic hormone-induced loading of diacylglycerol onto HDLp-A, using the analytical ultracentrifuge and gel filtration, suggests that a lipoprotein of density intermediate between HDLp-A and LDLp was formed transiently. Analysis of lipoproteins separated by density gradient ultracentrifugation in the course of the loading process indicates that apoLp-III is added more rapidly than diacylglycerol and that it changes its conformation on the surface as more diacylglycerol is added. Taken together with the known properties of apoLp-III, a prolate ellipsoid with an axial ratio of 3, we suggest that initially apoLp-III adds to the expanded hydrophobic surface of the lipoprotein with its short axis parallel to the surface and that apoLp-III subsequently unfolds to cover a greater area of hydrophobic surface. Exchange experiments with labeled apoLp-III showed that the two apoLp-III molecules in HDLp-A do not exchange with free apoLp-III, even when the lipoprotein passed through a loading and unloading cycle, suggesting a structural role for apoLp-III in HDLp-A.
D000818:Animals; D001053:Apolipoproteins / Q000502:physiology*; D002352:Carrier Proteins / Q000378:metabolism*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002850:Chromatography, Gel; D004075:Diglycerides / Q000378:metabolism; D006458:Hemolymph / Q000378:metabolism; D007700:Kinetics; D007915:Lepidoptera / Q000378:metabolism*; D008074:Lipoproteins / Q000378:metabolism*; D008297:Male; D008970:Molecular Weight; D009036:Moths / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism; D014461:Ultracentrifugation
D001053:Apolipoproteins; D002352:Carrier Proteins; D004075:Diglycerides; D008074:Lipoproteins; D010743:Phospholipids; C042470:apolipophorin III; C032830:lipophorin