Interaction of lipoprotein lipase with phospholipid vesicles: effect on protein and lipid structure.
The interaction of lipoprotein lipase (LpL) and a nonhydrolyzable phosphatidylcholine, 1,2-ditetradecyl-rac-glycero-3-phosphocholine (C14-ether-PC), has been studied by several physical methods. Analysis of the circular dichroic spectrum of LpL gave the following fractional conformation: 35% alpha-helix, 30% beta-pleated sheet, and 45% remaining structure. No significant change in the circular dichroic spectrum of LpL was observed on addition of C14-ether-PC vesicles. The quenching of LpL fluorescence by acrylamide and iodide ion was decreased only slightly by addition of C14-ether-PC vesicles. Addition of LpL to sonicated C14-ether-PC vesicles containing entrapped carboxyfluorescein caused the release of less than 15% of the vesicle contents in 20 min, indicating that the enzyme did not disrupt the structure of the lipid. In contrast, greater than 80% of the vesicle contents were released with the addition of apolipoprotein A-I to an identical vesicle preparation. The temperature dependence of the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene incorporated into C14-ether-PC vesicles was not significantly altered by the addition of LpL. When LpL is added to vesicles, the bilayer structure of the vesicles is not disrupted as observed by freeze-fracture electron microscopy. However, at low ionic strength (0.1-0.25 M NaCl) significant aggregation of intact vesicles is observed by light scattering and electron microscopy. Vesicle aggregation is prevented and reversed by 1 M NaCl and by heparin. These data demonstrate that LpL binds to the surface of a lipid interface, without dramatic changes in lipid bilayer or protein structure.
D000818:Animals; D002417:Cattle; D002942:Circular Dichroism; D004134:Dimyristoylphosphatidylcholine / Q000378:metabolism*; D005260:Female; D005452:Fluoresceins; D005614:Freeze Fracturing; D007700:Kinetics; D008071:Lipoprotein Lipase / Q000378:metabolism*; D008081:Liposomes; D008854:Microscopy, Electron; D008892:Milk / Q000201:enzymology; D008968:Molecular Conformation; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
D005452:Fluoresceins; D008081:Liposomes; C024098:6-carboxyfluorescein; D008071:Lipoprotein Lipase; D004134:Dimyristoylphosphatidylcholine