Changes in the density distribution of pig high density lipoproteins during incubation in vitro. Influence of esterified cholesterol transfer activity.

Pig plasma, depleted of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) by ultracentrifugation, was incubated in vitro at 37 degrees C in the presence of 0.002 M parachlormercuriphenyl sulfonate, a chemical inhibitor of lecithin: cholesterol acyltransferase. Incubation resulted in a consistent and significant shift in the density distribution of esterified cholesterol within the high density lipoprotein (HDL) fraction. After 24 h of incubation the esterified cholesterol in HDL of density greater than 1.125 g/ml was reduced by a mean of 28%, while that in HDL of density less than 1.125 g/ml was increased by a mean of 77%. Both the rate and the magnitude of this redistribution were markedly enhanced when the incubation mixture contained rabbit lipoprotein-free plasma, a rich source of the esterified cholesterol transfer protein. When pig plasma, depleted of VLDL and LDL, was subjected to rate zonal ultracentrifugation, the HDL in non-incubated samples eluted as a single symmetrical peak in a position comparable to that of human HDL3. After 24 h of incubation at 37 degrees C the density of the HDL was reduced, with a peak midway between the positions of human HDL3 and HDL2. When VLDL- and LDL-depleted pig plasma were mixed with human lipoprotein-free plasma (also a source of esterified cholesterol transfer activity) and incubated for 24 h at 37 degrees C, there was an even greater reduction in the density of pig HDL, which now eluted in a position comparable to that of human HDL2. It was concluded that the capacity of the esterified transfer protein to redistribute esterified cholesterol between different lipoprotein particles may be of importance in the process of HDL interconversion.