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

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Abstract
Induced circular dichroism (CD) of .beta.-carotene was used to study the physical state in the cores of 3 classes of triglyceride-rich lipoproteins from human serum: intermediate-density lipoproteins (IDL) (1.006 < d < 1.019 g/ml) and subfractions of the d < 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 wk. The .beta.- and pre-.beta.-migrating d < 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.degree. and 30.degree. C, 4 subfractions of the .beta.-migrating d < 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 < 1.006 g/ml lipoproteins showed no detectable induced CD of .beta.-carotene. The cores of triglyceride-rich lipoproteins can exist in some ordered state between 7.degree. and 30.degree. C if they have a relatively low ratio of triglycerides to cholesteryl esters (mass ratio < 1.6) and relatively small particle diameter (< 60 nm).