Redistribution of cholesterol and β-sitosterol between Intralipid and rat plasma lipoproteins and red blood cells in vivo

Male Wistar rats were injected intravenously with 2 mL of Intralipid containing 7.5 × 10 5 counts per minute (cpm) [ 14 C]cholesterol and 7.5 × 10 5 cpm β-[ 3 H]sitosterol. Blood was withdrawn immediately and at 5, 10, 20, 60,120, and 1440 min after injection from different animals. Plasma and red cells were separated and washed by conventional centrifugation, while lipoprotein density classes corresponding to chylomicrons, very low (VLDL), low (LDL), and high density lipoproteins (HDL) were isolated by ultracentrifugation. Total lipid and sterol compositions were determined by thin-layer chromatography in combination with gas–liquid chromatography, whereas radioactivity was measured by scintillation counting. The ratio of [ 14 C]cholesterol/β-[ 3 H]sitosterol rose from 1 to 3.65 in the plasma VLDL fraction, whereas that in the LDL and HDL fractions were equilibrated at about 2, following an initial transient increase in favour of cholesterol. The appearance and disappearance of the radioactivity from LDL and HDL fractions exhibited precursor–product relationship owing probably to the conversion of the Intralipid into an intermediate lipoprotein-X-like particle, which possesses a density similar to that of LDL. The radioactive cholesterol and β-sitosterol were incorporated into the red blood cell membranes at nearly similar initial rates, while at later times the incorporation of cholesterol was much preferred.