Characterization of high-density lipoprotein binding sites in rat liver and testis membranes

Abstract

The binding of human 125I-labeled HDL 3, to purified rat liver and testis plasma membranes was studied. About 50–65% of the total HDL binding in these membranes was abolished by 1% bovine serum albumin in the incubation medium. The remaining albumin-insensitive binding sites, determined in the presence of albumin were associated with plasma membranes; a good correlation was found between the 125I-labeled HDL 3 binding and the 5'-nucleotidase activities of the membrane fractions. The binding sites in these tissues were saturable, specific for HDL (not competed for by LDL) and had similar affinities for 125I-labeled HDL 3 ( K d , 11.8 μg protein/ml for liver and 12.7 μg protein/ml for testis membranes); the maximum binding capacity of the testis membranes was higher (1.3 vs. 0.7 μg protein/mg membrane protein). Egg phosphatidylcholine complexes of both human apolipoprotein A-I and apolipoprotein C's competed for the HDL-binding sites, but phosphatidylcholine vesicles alone did not. Chemical modification of the lysine and arginine residues of apolipoproteins did not affect the interaction of HDL 3 with its binding sites. Despite the fact that the HDL-binding sites in these tissues are not specific for apolipoprotein A-I, they may have important physiological roles in lipid transport, as they appear to recognize apolipoprotein-phospholipid complexes.