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

The binding of human 125I-labeled HDL3 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 HDL3 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 HDL3 (Kd, 11.8 micrograms protein/ml for liver and 12.7 micrograms protein/ml for testis membranes); the maximum binding capacity of the testis membranes was higher (1.3 vs. 0.7 microgram 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 HDL3 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.