Motion and surface accessibility of spin-labeled lipids in a model lipoprotein containing cholesteryl oleate, dimyristoylphosphatidylcholine, and apolipoprotein E

Abstract

A series of spin-labeled phosphatidylcholines (PCs) and cholesteryl esters (CEs) bearing the paramagnetic 2,2-dimethyloxazolidinyl-1-oxy (doxyl) group at fatty acyl carbon C5'', C12'', or C16'' were used to study acyl chain motions in the polar surface shell and hydrophobic core domains of microemulsion (ME) particles cholesteryl oleate and dimyristoylphosphatidylcholine (DMPC), and of particles with apolipoprotein E (apoE) bound to their surfaces. Electron paramagnetic resonance data obtained with the doxyl-labeled PCs indicated a gradient of motion in the ME surface monolayer similar to that observed with the same probes in a bilayer. The 5- and 12-doxyl-CEs clearly demonstrated a higher degree of order for the cholesteryl ester rich core than the corresponding doxyl-PCs showed for the phospholipid-rich surface over the entire range 10-60.degree. C. The temperature dependencies of spectra of the 16-doxyl-CE in the core and PC in the surface of the ME were almost identical, suggesting that there was no sharp boundary between core and surface domains. None of the probes detected either the surface phospholipid transition (31.degree. C) or the cholesteryl ester core transition (46.degree. C) measured previously by differential scanning calorimetry and 13C nuclear magnetic resonance. Binding of apoE to spin-labeled DMPC vesicles increased the order of the 5''-position of the sn-2 acyl chain over the range 15-33.degree. C; the thermal transition was broadened and its midpoint elevated. The effect of protein binding was not as striking for the ME particles. In separate studies, the rates of ascorbate-induced reduction of the nitroxyl moiety in ME labeled with either 5-doxyl-PC of 5-doxyl-CE were measured to determine the accessibility of each lipid type to the aqueous phase and the core .fwdarw. surface mobility of the nonpolar lipids. Reduction of 5-doxyl-PC in the ME was monophasic; the rates were comparable to those of 5-doxyl-CE in vesicles, but much lower than those of 5-doxy-PC in vesicles. This result indicated that the C5''-position of the sn-2 acyl chain of PC in the microemulsion was less accessible (by bulk water molecules) than the corresponding position in the vesicle. Reduction of 5-doxyl-CE in the ME was also monophasic and dependent on ascorbate concentration at every temperature studied. Thus, CE movement from the core to the surface was more rapid than the rate of doxyl group reduction. Calculations based on these results suggested that a significantly larger fraction of CE may be present in the ME surface monolayer than in the vesicle bilayer.