Lipoprotein-X: Carbon-13 Nuclear Magnetic Resonance Studies on Native, Reconstituted, and Model Systems

Lipoprotein-X (LP-X), a lipoprotein isolated from human cholestatic plasma by ethanol-acetate precipitation and zonal ultracentrifugation, has been studied by ¹³C NMR at 67.9 MHz. Spectra of LP-X and its three subfractions are markedly different from those of normal human high-density lipoprotein₃ (HDL₃) or low-density lipoprotein (LDL). Spectra of LP-X are characterized by the presence of unusually broad resonance lines, especially those attributable to C6 of unesterified cholesterol (160-260 Hz) and to Cβ of phospholipid glyceride (240-290 Hz). In contrast, the CH₂O, CH₂N, and N(CH₃)₃ choline resonances have line widths comparable to those of normal LDL and HDL₃. For the subfraction LP-X₁, spin-lattice relaxation times (T₁) of the fatty acyl olefin resonances at 129.8 and 128.0 ppm and of the unesterified cholesterol C6 at 120.1 ppm were measured to be 675, 766, and 162 ms, respectively. These times are comparable to those measured for the corresponding resonances in single bilayer vesicles whose lipid composition approximates that of LP-X. The three LP-X subfractions isolated by zonal ultracentrifugation gave spectra which are identical, within experimental error, as judged qualitatively from their appearance and quantitatively from the line widths of selected resonances. In addition, ¹³C NMR spectra of sonicated total LP-X lipids are similar to spectra of the intact native lipoprotein. This study suggests (a) that motions of lipids in LP-X as probed by ¹³C NMR are similar to the motions of lipids found in model vesicular systems, (b) that the motions of the cholesterol rings and phospholipid fatty acyl chains are significantly more restricted in LP-X than in HDL₃ and LDL, and (c) that the motions of the phosphoryl moieties in all three systems are similar.