Thomas L. Innerarity,* Robert E. Pitas, and Robert W. Mahley abstract: Previous equilibrium and kinetic studies have shown that cholesterol-induced high-density lipoproteins (HDLC), which contain only the arginine-rich (E) apoprotein (apo-E), have an affinity for normal human fibroblasts that was 20 times that of human low-density lipoprotein (LDL). Also, 4 times as many LDL particles as HDLC particles were required to saturate the normal human fibroblast surface re-ceptors. The most feasible explanation was that each HDLC particle bound to approximately four receptors. Here we considered two other possibilities:(a) that HDLC bind to one receptor, suppressing three other receptors and (b) that HDLC and LDL bind to different but adjacent receptors, such that binding of either blocksthe binding of the other. The pos-sibility of suppressed receptors proved unlikely because cells that had HDLC bound and then removed could still bind the full complement of LDL. The possibilityof blocked adjacent receptors was examined by using cell strains with genetic mutations in the LDL receptor locus. If the receptors for LDL and apo-E HDLC were different, a mutation that altered the binding of one would not necessarily alter the binding of the other. The bindingof LDL and HDLC to the mutant cells was essentially the same as to normal cells, with equilibrium dis-sociation constants (Kd) of 2.6 X 10~ 9 and 0.13 X 10™ 9 M, respectively. Though the total number of receptors available in the mutant cells from patients with the heterozygous form of familial hypercholesterolemia was only 50% of normal cells, I-iow-density lipoproteins (LDL) 1 and certain cholesterolinduced atypical high-density lipoproteins (HDLC) bind to specific lipoprotein receptors on the surface of normal human fibroblast cells in culture and are subsequently internalized and degraded (Brown & Goldstein, 1976a; Mahley et al., 1978; Mahley & Innerarity, 1978). The HDLC induced in dogs and swine by cholesterol feeding contain high concentrations of the arginine-rich apoprotein (apo-E) that mediates their re-ceptor binding (Innerarity & Mahley, 1978), just as the B apoprotein (apo-B) mediates the binding of LDL (Mahley et al., 1977a; Shireman et al., 1977). Significant differences have been noted between LDL containing apo-Band HDLC containing apo-E in their inter-action with the cell surface binding sites. Competitive binding studies by Innerarity & Mahley (1978) performed at 4 C showed that apo-E HDLC, an HDLC that contains only the E apoprotein, exhibit a 100-fold greater ability than LDL to displace [125I] LDLfrom high-affinity sites on the cell surface of normal human fibroblasts. This enhanced binding of apo-E