Human high density lipoprotein (HDL 3) binding to rat liver plasma membranes

Abstract

The binding of human 125I-labeled HDL 3 to purified rat liver plasma membranes was studied. 125I-labeled HDL 3 bound to the membranes with a dissociation constant of 10.5 μg protein/ml and a maximum binding of 3.45 μ g protein/mg membrane protein. The 125I-labeled HDL 3-binding activity was primarily associated with the plasma membrane fraction of the rat liver membranes. The amount of 125I-Iabeled HDL 3 bound to the membranes was dependent on the temperature of incubation. The binding of 125I-labeled HDL 3 to the rat liver plasma membranes was competitively inhibited by unlabeled human HDL 3, rat HDL, HDL from nephrotic rats enriched in apolipoprotein A-I and phosphatidylcholine complexes of human apolipoprotein A-I, but not by human or rat LDL, free human apolipoprotein A-I or phosphatidylcholine vesicles. Human 125I-labeled apolipoprotein A-I complexed with egg phosphatidylcholine bound to rat liver plasma membranes with high affinity and saturability, and the binding constants were similar to those of human 125I-labeled HDL 3. The 125I-labeled HDL 3-binding activity of the membranes was not sensitive to pronase or phospholipase A 2 however, prior treatment of the membranes with phospholipase A 2 followed by pronase digestion resulted in loss of the binding activity. Heating the membranes at 100° C for 30 min also resulted in an almost complete loss of the 125I-labeled HDL 3-binding activity.