Differentiation of binding sites on reconstituted hepatic scavenger receptors using oxidized low-density lipoprotein

Reduced hepatic membrane receptors for acetylated low-density lipoprotein (acetyl-LDL) and maleylated BSA (Mal-BSA) with apparent molecular masses of 35 kDa, 85 kDa and 15 kDa have been extracted from rat liver and separated by affinity chromatography as described by us previously [Ottnad, Via, Sinn, Freidrich, Ziegler & Dresel (1990) Biochem. J. 265, 689-698]. Binding of these three reduced scavenger receptors to oxidatively modified LDL has been now examined. Competition studies with receptor-phosphatidylcholine complexes and 131I-acetyl-LDL and 131I-Mal-BSA as ligands were conducted. Mal-BSA, acetyl-LDL and fully oxidized LDL were used as competitors, and differentiated in the three receptors three types of binding site: a class I binding site for acetyl-LDL, Mal-BSA and fully oxidized LDL; a class II binding site recognizing only 131I-Mal-BSA and class III binding sites recognizing 131I-Mal-BSA and fully oxidized LDL. The results of competition studies with mildly oxidized LDL and polyadenylic acid demonstrated that the binding sites might be even more heterogenous. Thus there is evidence that the reconstituted receptors either have several binding sites for each of the various ligands or are functionally different, despite the fact that they do not differ in their apparent molecular masses.