STUDIES ON EQUILIBRATION OF RADIOISOTOPIC CHOLESTEROL WITH HUMAN SERUM LIPOPROTEIN CHOLESTEROL IN VITRO

1. 1. The process of equilibration of radioisotopic cholesterol with human serum lipoprotein cholesterol in vitro has been examined. The initial step is the formation in serum of dispersions of large (> 0.45 μ) or small (0.1-0.45 μ) particles by addition of an ethanolic solution of cholesterol. The smaller particles exhibit the electrophoretic mobility of β-lipoprotein and are largely precipitable by anti-β-lipoprotein serum. The larger particles are also precipitable by antiserum and contain unesterified cholesterol as their only detectable lipid. These results, along with previously reported ultracentrifuge flotation data, favor a structure for both particles consisting of a cholesterol or cholesterol-protein core surrounded by a monolayer of β-lipoprotein. 2. 2. A lipoprotein precipitation procedure, capable of precipitating particles at high specific activity, has been used to test for completeness of equilibration. Radioactivity in large-particle form fails to equilibrate after incubation at 37° for 20 h, while about 83% of the unesterified labeled cholesterol in small particle form does so as judged by specific activity determinations. Large particle cholesterol equilibrates at a single exponential rate of only 2.0%/h and small particle at 11.3%/h. 3. 3. Unequilibrated small-particle [3H] cholesterol transfers to human erythrocytes in vitro at a rate similar to that reported for the exchange of lipoprotein cholesterol with erythrocyte cholesterol. Large-particle 3H is rapidly adsorbed by the cells. 4. 4. Labeled cholesteryl esters, formed during incubation of small or large particles, equilibrate among serum lipoproteins. A recently reported process of net transfer of cholesteryl esters and other lipids in human serum during incubation in vitro could explain this result. The same mechanism might account for equilibration of unesterified cholesterol, through transfer of a cholesterol-cholesteryl ester core. Evidence for molecular exchange in vitro with lipoprotein cholesterol is not conclusive. © 1968.