Structure and motion of phospholipids in human plasma lipoproteins. A phosphorus-31 NMR study

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Abstract
The structure and motion of phospholipids in human plasma lipoproteins have been studied by using 31P NMR. Lateral diffusion coefficients, DT, obtained from the viscosity dependence of the 31P NMR line widths, were obtained for very low density lipoprotein (VLDL), low-density lipoprotein (LDL), high-density lipoproteins (HDL2, HDL3), and egg PC/TO microemulsions at 25.degree. C, for VLDL at 40.degree. C, and for LDL at 45.degree. C. At 25.degree. C, the rate of lateral diffusion in LDL (DT = 1.4 .times. 10-9 cm2/s) is an order of magnitude slower than in the HDLs (DT = 2 .times. 10-8 cm2/s). At 45.degree. C, DT for LDL increases to 1.1 .times. 10-8 cm2/s. In contrast, DT for VLDL increases only slightly going from 25 to 40.degree. C. The large increase in diffusion rate observed in LDL occurs over the same temperature range as the smectic to disordered phase transition of the core cholesteryl esters, and provides evidence for direct interactions between the monolayer and core. In order to prove the orientation and/or order of the phospholipid head-group, estimates of the residual chemical shift anistropy, .DELTA..sigma., have been obtained for all the lipoproteins and the microemulsions from the viscosity and field dependence of the 31P NMR line widths. For VLDL and LDL, the anisotropy is 47-50 ppm at 25.degree. C, in agreement with data from phospholipid bilayers. For the HDLs, however, significantly larger values of 69-75 ppm (HDL2) and > 120 ppm (HDL3) were obtained. These results suggest differences in the orientation and/or ordering of the head-group in the HDLs. The dynamic behavior of the phosphate moiety in LDL and HDL3 had been obtained from the temperature dependence of the 31P spin-lattice relaxation rates. Values of the correlation time for phosphate group reorientation and the activation energy for the motion are nearly identical in LDL and HDL3 and are similar to values obtained for phospholipid bilayers. This argues agonist long-lived interactions being the source of either the slow diffusion in LDL or the altered head-group orientation in the HDLs.