Detection of LDL Receptors in Liver Membranes by Ligand Blotting with Biotin-Modified Plasma Lipoproteins

Abstract

Low density lipoprotein receptor (LDL-receptor) activity in hepatic tissues is difficult to identify and measure accurately by ligand-binding assays that depend upon separation of bound and free ligand by membrane filtration (Schneider et al., 1980) or centrifugation (Basu et al., 1978). With liver membranes, the proportion of non-specific binding relative to specific, saturable binding is high and saturable binding of LDL is less sensitive to EDTA, a potent inhibitor of LDL receptor activity, than in other tissues (Harders-Spengel et al., 1982). The LDL receptor protein in human skin fibroblasts and bovine adrenal cortex has been demonstrated by ligand blotting with LDL (Daniel et al., 1983). The native LDL receptor exhibits Ca⁺⁺ dependent saturable binding of lipoproteins containing apoB and apoE and these properties are retained in the blotted protein if detergent-solubilised receptor is subjected to SDS-PAGE under non-reducing conditions. We have recently described a new method for the detection of bound lipoproteins based on the high affinity streptavidin-biotin interaction (Wade et al., 1985), and in this paper we report that a protein of MW approx. 130,000–150,000 can be detected in extracts of both adult dog liver and normal rat liver by ligand blotting with biotin-LDL. Binding of the lipoprotein was saturable and EDTA-sensitive, and thus the protein has the properties of the LDL-receptor of human skin fibroblasts or bovine adrenal cortex. Rat chylomicron remnants and rabbit ßVLDL, both modified with biotin, were also bound by the LDL-receptor protein in bovine adrenal cortex, dog liver and rat liver.