Plasma cholesteryl esters turnover in man: comparison of in vivo and in vitro methods

The rate of formation of plasma cholesteryl esters was determined by both in vivo and in vitro methods in 15 subjects. In vivo production of plasma cholesteryl esters was calculated from the specific activity slopes of plasma free and esterified cholesterol after an intravenous injection of [3H] mevalonic acid or [3H] cholesterol incorporated in plasma lipoproteins. In vitro production of cholesteryl esters was estimated by measuring lecithin:cholesterol acyltransferase activity in plasma. Lecithin:cholesterol acyltransferase was estimated by incubating the subjects' own plasma for 1 h at 37 degrees C. The plasma sample used for incubation was collected 2 h after the injection of radioactive precursor (when radioactivity of esterified cholesterol was very low relative to that in free cholesterol and the specific activity of free cholesterol in all of the major plasma lipoprotein classes was identical). The mean value for the production of plasma cholesteryl esters obtained by in vivo method was 126.2 +/- 41.9 mg/h, and it was not significantly different from the mean of 110.5 +/- 25.5 mg/h calculated from the results of in vitro method. The values obtained by the two methods in all 15 subjects had an excellent correlation (r = 0.93). It was found that in normotriglyceridemic subjects the values obtained by the two methods wwere essentially identical, but in hypertriglyceridemic subjects the values obtained by the in vitro method were consistently somewhat lower than the obtained by the in vivo method.