Liver uptake of chylomicron remnants with high and low apoprotein E:C ratios

The ability of the isolated perfused rat liver to differentiate between chylomicrons and remnants with either high or low apoprotein E:C ratios was investigated. Remnants were prepared in hepatectomized rats injected with chylomicrons double-labeled with [3H]cholesterol and 14C-labeled fatty acids. By densitometric scanning of polyacrylamide gels, the apoprotein E:C ratio of the chylomicrons was 0.8 and that of the remnants was 1.5. When livers were perfused with these lipoproteins in a recirculatory system for 4 min, uptake of remnants was about 3-fold greater than that of chylomicrons. Preparation of remnants as well as chylomicrons with a low apoprotein E:C ratio was achieved by (i) removal of all apoproteins from the surface of the lipoproteins by trypsin digestion, followed by (ii) transfer of soluble apoproteins from serum lipoproteins to the apoprotein-free particles. The apoprotein E:C ratio of the reconstituted lipoproteins was decreased from 1.5 to 0.3 for remnants and from 0.8 to 0.2 for chylomicrons. In spite of these changes in apoprotein E:C ratios, the hepatic uptake of the reconstituted lipoproteins with low apoprotein E:C ratios was similar to their unmodified controls. These results indicate that the hepatic discrimination between chylomicrons and remnants is not determined by the relative amounts of apoproteins E and C on the surface of the lipoproteins.