PHYSICOCHEMICAL STUDIES OF 2 MAJOR SUBPOPULATIONS OF LOW-DENSITY LIPOPROTEINS BY DIFFERENTIAL SCANNING CALORIMETRY AND NMR-SPECTROSCOPY

Two subpopulations, layer 2 (density 1.025-1.029 g/ml) and layer 3 (density 1.032-1.043 g/ml) of low density lipoproteins (LDL) were isolated from fresh human plasma of normal lipidaemic subjects by density gradient ultracentrifugation. Chemical analyses demonstrated the ratios of triglyceride/cholesterol ester decreased with increasing densities of subfractions. These subfractions together with triglyceride-rich lipoproteins (layer 1, density < 1.019 g/ml) were subjected to physicochemical studies by differential scanning calorimetry (d.s.c.) and nuclear magnetic resonance (n.m.r.) spectroscopy. The average transition temperature (Tt) of layer 2 was 34.20±0.83°C and that of layer 3 was 37.25±0.35°C. In addition, many of the layer 3, but not layer 2 and layer 1, samples showed structural alteration and gave rise to an average Tt of 39.18±1.24°C. The structural alteration could be detected with polarizing light microscopy showing birefringent spherulites at body temperature. The peak Tt values obtained by d.s.c. were in good agreement with those by n.m.r. spectroscopy. These results demonstrate the physicochemical heterogeneity within the LDL density region and suggest that layer 3 subpopulation is much more labile than the others. © 1990.