High-resolution NMR spectra of high-density serum lipoproteins

Abstract

1. High-resolution 220 Mcycles/sec and 100 Mcycles/sec proton resonance spectra have been observed with high-density serum lipoproteins (HDL₂ and HDL₃) dissolved in ²H₂O and examined at different temperatures. Signals associated with the lipid and the amino acids of the protein are prominent. 2. To aid interpretation of the spectra and the nature of the lipid-protein interactions involved, spectra were also obtained with (a) the apoprotein obtained by removal of the lipoprotein lipids; (b) the lipids extracted from the lipoproteins; (c) a reconstituted lipoprotein dissolved in ²H₂O. For comparison purposes the spectra of lysozyme, β-casein and cholesterol oleate were also examined. 3. The lipid in the lipoproteins is in an extremely fluid condition and probably in a magnetically isotropic environment. The spectrum of the lipid is similar to that which is observed when lipids are dissolved in organic solvents, or alternatively, dispersed in water by bile salts or detergents or in a sonicated form. 4. The cholesterol ring system of the cholesterol esters appears to be slightly restricted at the lower temperatures, so that the methyl signals associated with the cholesterol ring structure do not show signals. However, on increasing the temperature, these signals become apparent. 5. The organization of lipids and proteins in these lipoproteins is much looser than occurs in erythrocyte membranes. 6. After reconstitution of the apoprotein with phospholipid a similar spectrum to that of the original lipoprotein is observed. The reconstituted lipoprotein, however, appears to have a somewhat looser structure than the original lipoprotein. No marked differential broadening effect is observed with any of the signals associated with this phospholipid-protein complex.