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

Abstract

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 125I-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 μg/ml; at 37°, 750-2199 ng/mg was bound and internalized. The binding of apo B greatly exceeded the amount of 125I-labeled LDL bound at 4° and 37° in the same experiment. Fibroblasts from a subject homozygous for hyper-β-lipoproteinemia showed minimal binding of 125I-labeled LDL, consistent with the absence of the cellular LDL receptor. Such cells alo had depressed binding of 125I-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 125I-labeled apo B to lymphocytes paralleled the binding of 125I-labeled LDL. Unlabeled LDL and apo B-albumin complex both competitively inhibited the binding of 125I-labeled apo B and 125I-labeled LDL to fibroblasts. When labeled LDL was incubated with fibroblasts for 6 hr at 37°, it underwent cellular internalization and degradation, as measured by the release of 125I-labeled fragments into the medium. This degradation was inhibited by unlabeled apo B. Conversely, 125I-labeled apo B also was internalized and degraded by fibroblasts, and this process ws 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.