Thermal behavior of cores of human serum triglyceride-rich lipoproteins: a study of induced circular dichroism of beta-carotene.

Induced circular dichroism (CD) of beta-carotene has been used to study the physical state in the cores of three classes of triglyceride-rich lipoproteins from human serum: intermediate-density lipoproteins (IDL) (1.006 less than d less than 1.019 g/mL) and subfractions of the d less than 1.006 g/mL lipoproteins of beta and pre-beta electrophoretic mobility. Effects on the physical state in the cores attributable to the ratio of triglycerides to cholesteryl esters and particle diameters were assessed by comparing the temperature-dependent CD spectra of beta-carotene with those of low-density lipoproteins (LDL). Lipoproteins were prepared from serum by sequential ultracentrifugation after the donors were given supplemental dietary beta-carotene (60 mg/day) for 2 weeks. The beta- and pre-beta-migrating d less than 1.006 g/mL lipoproteins were separated by starch block electrophoresis and were then individually separated into subfractions by agarose gel filtration chromatography. Between 7 and 30 degrees C, four subfractions of the beta-migrating d less than 1.006 g/mL lipoproteins and IDL exhibited reversible, temperature-dependent induced CD of beta-carotene, with contours similar to those of LDL but with smaller magnitudes and much broader transitions of the CD bands than those of LDL. In contrast, subfractions of the pre-beta-migrating d less than 1.006 g/mL lipoproteins showed no detectable induced CD of beta-carotene. These results show that the cores of triglyceride-rich lipoproteins can exist in some ordered state between 7 and 30 degrees C if they have a relatively low ratio of triglycerides to cholesteryl esters (mass ratio less than 1.6) and relatively small particle diameter (less than 60 nm).