LOW-DENSITY-LIPOPROTEIN BINDING ASSAY USING THE CALF ADRENOCORTICAL LOW-DENSITY-LIPOPROTEIN RECEPTOR

The low-density lipoprotein (LDL) receptor was purified to a semipure solubilized form from calf adrenocortical tissue. This receptor was fo und to be a suitable substitute for the human LDL receptor for studyin g human LDL binding. The apparent dissociation constant of the recepto r from calf adrenocortical cells, using human LDL as the ligand, was f ound to be 8.8 +/- 1.0 mu g I-125-LDL/mL, similar to that reported for the human LDL receptor (4-10 mu g LDL/mL). The calf adrenocortical LD L receptor demonstrated specificity toward human lipoprotein fractions that was identical with that of the human LDL receptor. A competitive binding assay was optimized using the semipurified solubilized calf a drenocortical receptor. This facilitated the study of nonenzymatically glycosylated human LDL by a binding assay that is much simpler and fa ster than previous studies, which used intact cultured cells. The pres ent assay requires only a I-h incubation of LDL with the receptor and a simple filtration procedure to remove unbound LDL. Using the present assay, it was shown that nonenzymatic glycosylation of LDL on the ord er of what is seen in diabetics, that is, modification of 2-5% of lysi ne residues, caused a decreased ability of the LDL to bind to the rece ptor.