Influence of lipoprotein concentration on the exchanges of triacylglycerol between rabbit plasma low density and high density lipoproteins.

Molecular exchanges of triacylglycerol between rabbit serum low density lipoproteins (LDL) and high density lipoproteins (HDL) have been studied in 37 degrees C incubations performed in the presence of rabbit lipoprotein-free serum as a source of the triacylglycerol transfer protein. The molar rate of exchange of triacylglycerol between the two fractions increased with increasing incubation concentrations of LDL but was decreased as the HDL concentration was increased. When the concentration of both LDL and HDL was increased in parallel there was an increase in the molar rate of triacylglycerol exchange between the two fractions which flattened at higher concentrations, suggesting that the process was saturable. Fractionation of rabbit lipoprotein-free serum on a column of Sephadex G-200 resulted in the elution of the triacylglycerol transfer activity in a single peak. Addition of LDL to the lipoprotein-free serum had no effect on the position of elution of the triacylglycerol transfer activity. Addition of HDL, however, resulted in an elution of the transfer activity as two peaks, one in the original position and the other in the same position as HDL. The results of the kinetic studies have been interpreted in terms of a binding of the triacylglycerol transfer protein to HDL, but not to LDL.