FLUORESCENCE STUDY OF THE MOTIONAL STATES OF CORE AND SURFACE-LIPIDS IN NATIVE AND RECONSTITUTED LOW-DENSITY LIPOPROTEINS

Low density lipoproteins (LDL) consist of an apolar core of cholestero l esters and triglycerides surrounded by a monolayer of phospholipid, cholesterol, and a single molecule of apolipoprotein B (apoB-100). To determine the influence of core and surface constituents on the surfac e order of LDL, we have measured core and surface order parameters for native LDL, and reconstituted LDLs (rLDL) whose apolar core lipids we re extracted and replaced with either cholesterol oleate (CO) or triol ein (TO). Order parameters were measured by fluorescence depolarizatio n of diphenylhexatriene (DPH), which is located primarily in the core, and of trimethylammoniumdiphenylhexatriene (TMA-DPH), which is anchor ed at the water-phospholipid interface. DPH order parameters for LDL r econstituted with TO (r-TOLDL) were much lower than those for LDL re constituted with CO (r-COLDL), consistent with the physical properti es of TO, a nonviscous liquid at all temperatures studied, and CO, whi ch exists in a liquid crystalline or viscous liquid state at the tempe ratures studied. Although core cholesterol esters in rCOLDL and nati ve LDL undergo distinct order-disorder transitions, these transitions were not detected by DPH. This is most likely due to the difference be tween the time scale for end-over-end tumbling of cholesterol esters a nd the fluorescence lifetime of DPH. Despite the fact that the core li pids of r-COLDL were much more ordered than those of r-TOLDL, surf ace order parameters for both lipoproteins were similar. We conclude t hat the motional states of the core and surface lipids are relatively independent. Surface order parameters for native LDL were higher than those for reconstituted LDLs. This was attributed to the presence of u nesterified cholesterol in the surface of native LDL, and its absence in reconstituted LDL. Finally, the outer surface of r-COLDL was show n to be more ordered than that of unilamellar vesicles. We suggest tha t this is due to the presence of apoB-100 and neutral lipid molecules in the highly curved surface of LDL which reduce the motional freedom of surface phospholipids.