Natural Abundance l3 C Nuclear Magnetic Resonance Studies of Human Plasma High Density Lipoproteins*

High resolution proton-decoupled natural abundance ‘:‘C NMR spectra (at 15.18 MHz) of human plasma high density lipoproteins were recorded at 29”C, Yfi”C, and 43°C. Carbon 5 of unesterified cholesterol and the corresponding carbon of cholesteryl esters yield narrow, well resolved resonances. Values of nuclear Overhauser enhancements and integrated intensities for C-5 of free and esterified cholesterol, C-6 of cholesteryl esters, and the methyl groups on cationic nitrogen of phospholipids were measured. At 35”C, the ratio of integrated intensity of the C-5 resonance of cholesteryl ester to that of cholesterol, 51, is significantly different from the molar ratio of these species, 3:1, determined by chemical analysis. This result requires that at least two pools of cholesterol exist within the HDL particle and suggests that a substantial fraction of the cholesterol is sufficiently motionally restricted so as not to yield a detectable C-.5 resonance. Comparison of integrated intensities of peaks assignable to cholesteryl esters and phospholipids suggests that all or a large fraction of the cholesteryl esters contribute to the high density lipoprotein spectrum. The integrated intensities of peaks reflecting cholesterol and cholesteryl esters do not change over the temperature range 29-43”C, suggesting that no major structural reorganization of these components involving phase changes or formation of highly ordered domains occurs over this temperature range.