Kinetic studies of the transfer of esterified cholesterol between human plasma low and high density lipoproteins.

In vitro incubations (6 hr at 37 degrees C) of human low density lipoproteins (LDL), high density lipoproteins (HDL), and lipoprotein-free plasma revealed no significant net mass transfers of esterified cholesterol from either lipoprotein fraction to the other. Transfers of esterified [3H]cholesterol from LDL to HDL must therefore have represented a process of molecular exchange between the two fractions. In molar terms, the exchange increased with increasing incubation concentrations of LDL regardless of whether the HDL was increased in parallel, decreased, or kept constant. In direct contrast, with LDL kept constant, an increase in the concentration of HDL resulted in a decrease in the molar rate of exchange of esterified cholesterol between LDL and HDL. The data were then fitted to a mathematical model describing a physical model in which an esterified cholesterol transfer protein circulates in the plasma, interacting with lipoprotein particles into which it deposits and from which it picks up esterified cholesterol molecules. According to this model, to which the experimental data fit extremely well, the transfer protein had a much greater affinity for HDL than for LDL in a transfer process that was readily saturable by HDL but not by LDL.