Solubilization of the low density lipoprotein receptor

Abstract

The low density lipoprotein (LDL) receptor was solubilized from membranes of bovine adrenal cortex and cultured human cells by incubation with the nonionic detergent octyl-beta-D-glucoside. Receptor activity released into the 100,000 x g supernatant was assayed by a solid-phase procedure: an aliquot of the soluble extract was removed, the detergent was diluted below its critical micellar concentration, causing the receptor to precipitate as a lipid-protein aggregate; the precipitate was collected by centrifugation and incubated with (125)I-labeled LDL ((125)I-LDL); and the receptor-bound (125)I-LDL was separated from free (125)I-LDL by filtration. The (125)I-LDL binding site that was precipitated from the soluble extract of bovine adrenocortical membranes appeared to be the same as the functional LDL receptor of cultured bovine adrenocortical cells and human fibroblasts. It exhibited high affinity and specificity (affinity for LDL more than 200-fold greater than for acetylated LDL, methylated LDL, or high density lipoprotein), dependence on calcium, and susceptibility to destruction by Pronase. The amount of (125)I-LDL binding activity in solubilized membranes from cultured cells was proportional to the number of receptors on the surface of the intact cells. Thus, the number of solubilized receptors was 1/20th of normal in mutant fibroblasts from a subject with homozygous familial hypercholesterolemia and was 1/4th of normal in human epithelioid carcinoma A-431 cells when they were grown in the presence of 25-hydroxycholesterol plus cholesterol. While in the soluble form in the presence of octyl-beta-D-glucoside, the LDL receptor can be carried through several steps of purification.