Cholesteryl ester turnover in human plasma lipoproteins during cholestyramine and clofibrate therapy

The effects of cholestyramine and of clofibrate on the turnover rates of individual cholesteryl esters in whole human plasma and in each of the three classes of plasma lipoproteins have been studied. Four hyperlipidemic patients (two under treatment with each of the two drugs) were injected intravenously with cholesterol-(14)C, and serial plasma samples were collected after 3-4 hr, 8 hr, 24 hr, and 4-5 days. The plasma samples were separated into three classes of lipoproteins by ultracentrifugation. The cholesteryl esters and free cholesterol were isolated from each sample, and the specific radioactivity of the free and esterified cholesterol was determined. The specific radioactivity of each individual cholesteryl ester was then determined for each sample, by separately measuring the distribution of cholesterol mass and of radioactivity among four different cholesteryl ester groups, namely the saturated, mono-, di-, and tetra-unsaturated esters. In all subjects the plasma cholesteryl esters were metabolically heterogeneous, and could be divided into three pools corresponding to the three classes of plasma lipoproteins. High density lipoprotein (d > 1.063) cholesteryl esters showed the greatest fractional turnover rate, and low density lipoprotein (d 1.019-1.063) cholesteryl esters showed the smallest fractional turnover rate. In each subject the cholesteryl ester composition of the three classes of plasma lipoprotein was almost identical. Within each lipoprotein, and in whole plasma, all the different individual cholesteryl esters were found to turn over at the same fractional rate. In all respects these results were similar to those previously obtained with normal subjects. The results suggest that neither drug has a strongly selective effect on the turnover of one particular cholesteryl ester, or on the turnover or composition of the cholesteryl esters in one particular plasma lipoprotein.