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Solubilization of apolipoprotein B and its specific binding by the cellular receptor for low density lipoprotein. 1977

R Shireman, and L L Kilgore, and W R Fisher

Low density lipoprotein (LDL) and very low density lipoprotein (VLDL) bind specifically to a receptor on fibroblasts, and it has been postulated that the apoprotein of LDL (apo B) confers the specificity of cellular binding. This hypothesis has been tested in the present study with a watersoluble apo B-bovine serum albumin complex. The binding of (125)I-labeled apo B to cultured fibroblasts was temperature-dependent. Specific binding ranged between 183 and 859 ng/mg of cell protein at a concentration of 5 mug/ml; at 37 degrees , 750-2199 ng/mg was bound and internalized. The binding of apo B greatly exceeded the amount of (125)I-labeled LDL bound at 4 degrees and 37 degrees in the same experiment. Fibroblasts from a subject homozygous for hyper-beta-lipoproteinemia showed minimal binding of (125)I-labeled LDL, consistent with the absence of the cellular LDL receptor. Such cells also had depressed binding of (125)I-labeled apo B. Lymphocytes grown in lipoprotein-deficient medium demonstrated specific binding of LDL; however, freshly isolated lymphocytes did not show such binding. The binding of (125)I-labeled apo B to lymphocytes paralleled the binding of (125)I-labeled LDL. Unlabeled LDL and apo B-albumin complex both competitively inhibited the binding of (125)I-labeled apo B and (125)I-labeled LDL to fibroblasts. When labeled LDL was incubated with fibroblasts for 6 hr at 37 degrees , it underwent cellular internalization and degradation, as measured by the release of (125)I-labeled fragments into the medium. This degradation was inhibited by unlabeled apo B. Conversely, (125)I-labeled apo B also was internalized and degraded by fibroblasts, and this process was inhibited by LDL. These findings demonstrate that apo B binds specifically to the LDL receptor and that the cellular binding of LDL is determined by this apoprotein.

UI MeSH Term Description Entries
D008077 Lipoproteins, LDL A class of lipoproteins of small size (18-25 nm) and light (1.019-1.063 g/ml) particles with a core composed mainly of CHOLESTEROL ESTERS and smaller amounts of TRIGLYCERIDES. The surface monolayer consists mostly of PHOSPHOLIPIDS, a single copy of APOLIPOPROTEIN B-100, and free cholesterol molecules. The main LDL function is to transport cholesterol and cholesterol esters to extrahepatic tissues. Low-Density Lipoprotein,Low-Density Lipoproteins,beta-Lipoprotein,beta-Lipoproteins,LDL(1),LDL(2),LDL-1,LDL-2,LDL1,LDL2,Low-Density Lipoprotein 1,Low-Density Lipoprotein 2,LDL Lipoproteins,Lipoprotein, Low-Density,Lipoproteins, Low-Density,Low Density Lipoprotein,Low Density Lipoprotein 1,Low Density Lipoprotein 2,Low Density Lipoproteins,beta Lipoprotein,beta Lipoproteins
D011485 Protein Binding The process in which substances, either endogenous or exogenous, bind to proteins, peptides, enzymes, protein precursors, or allied compounds. Specific protein-binding measures are often used as assays in diagnostic assessments. Plasma Protein Binding Capacity,Binding, Protein
D002462 Cell Membrane The lipid- and protein-containing, selectively permeable membrane that surrounds the cytoplasm in prokaryotic and eukaryotic cells. Plasma Membrane,Cytoplasmic Membrane,Cell Membranes,Cytoplasmic Membranes,Membrane, Cell,Membrane, Cytoplasmic,Membrane, Plasma,Membranes, Cell,Membranes, Cytoplasmic,Membranes, Plasma,Plasma Membranes
D004591 Electrophoresis, Polyacrylamide Gel Electrophoresis in which a polyacrylamide gel is used as the diffusion medium. Polyacrylamide Gel Electrophoresis,SDS-PAGE,Sodium Dodecyl Sulfate-PAGE,Gel Electrophoresis, Polyacrylamide,SDS PAGE,Sodium Dodecyl Sulfate PAGE,Sodium Dodecyl Sulfate-PAGEs
D005347 Fibroblasts Connective tissue cells which secrete an extracellular matrix rich in collagen and other macromolecules. Fibroblast
D006801 Humans Members of the species Homo sapiens. Homo sapiens,Man (Taxonomy),Human,Man, Modern,Modern Man
D001053 Apolipoproteins Protein components on the surface of LIPOPROTEINS. They form a layer surrounding the hydrophobic lipid core. There are several classes of apolipoproteins with each playing a different role in lipid transport and LIPID METABOLISM. These proteins are synthesized mainly in the LIVER and the INTESTINES. Apolipoprotein
D001665 Binding Sites The parts of a macromolecule that directly participate in its specific combination with another molecule. Combining Site,Binding Site,Combining Sites,Site, Binding,Site, Combining,Sites, Binding,Sites, Combining
D012995 Solubility The ability of a substance to be dissolved, i.e. to form a solution with another substance. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 6th ed) Solubilities
D066298 In Vitro Techniques Methods to study reactions or processes taking place in an artificial environment outside the living organism. In Vitro Test,In Vitro Testing,In Vitro Tests,In Vitro as Topic,In Vitro,In Vitro Technique,In Vitro Testings,Technique, In Vitro,Techniques, In Vitro,Test, In Vitro,Testing, In Vitro,Testings, In Vitro,Tests, In Vitro,Vitro Testing, In

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