Pathways for the incorporation of esterified cholesterol into very low density and low density lipoproteins in plasma incubated in vitro.

In vitro incubations of human or pig plasma containing a tracer amount of [3H]cholesterol have been performed to determine which lipoprotein fractions are the immediate recipients of the esterified cholesterol formed in the reaction catalysed by lecithin: cholesterol acyltransferase. In pig plasma, which is deficient in activity of the protein which promotes transfer of esterified cholesterol between different lipoprotein fractions, 87-90% of the lecithin: cholesterol acyltransferase-derived esterified cholesterol was incorporated into the high density lipoprotein (HDL) fraction. In human plasma there was an initial recovery of more than 80% in HDL, although the proportion recovered in very low density lipoproteins (VLDL) and low density lipoproteins (LDL) became progressively greater with increasing duration of incubation, consistent with a transfer from an HDL -esterified cholesterol pool of increasing specific activity. Nevertheless, as in the pig plasma incubations, there was evidence that some 10-15% of the esterified cholesterol formed in the lecithin: cholesterol acyltransferase reaction was incorporated directly into human VLDL and LDL. In quantitative terms, however, it was found that most of the esterified cholesterol delivered to human VLDL and LDL was the result of transfers from HDL rather than as a direct incorporation from its site of synthesis.