Turnover of Individual Cholesterol Esters in Human Liver and Plasma*

There are several lines of evidence which sug-gest that the liver is a major source of plasma cho-lesterol esters. First, eviscerated rats cannot main-tain a normal concentration of cholesterol esters in the plasma (1); second, when isotopically la-beled mevalonic acid is administered to rats, the specific radioactivity of the hepatic cholesterol esters initially exceeds that of the plasma choles-terol esters (2); and third, the perfusion of livers from cholesterol-fed rats leads to an increased secretion of cholesterol esters from the liver into the perfusate (3). Despite this, the composition of cholesterol esters in the liver and plasma is dissimilar, both in the rat (2, 3) and in man (4, 5). This has raised the question of whether an additional source of plasma cholesterol esters exists outside the liver. A transesterification reaction between free cho-lesterol and the beta fatty acid of lecithin has been demonstrated in plasma and could account for a portion of the plasma cholesterol esters (6, 7). Simultaneous esterification of cholesterol in liver and plasma could lead to the formation of choles-terol esters of differing composition. Some of the factors that govern the formation and composition of cholesterol esters have been determined in the rat. The turnover of cholesterol esters in rat liver and plasma is heterogeneous. The fractional turnover rate of plasma cholesteryl oleate is greater than that of the other esters (2) and may be related to the high degree of specificity for cholesteryl oleate formation by liver enzymes (8). When rats are fed cholesterol with different fatty acids, cholesteryl oleate accumulates in the * Submitted for publication November 1, 1965; ac