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Rate and equilibrium constants for binding of apo e high density lipo protein c a cholesterol induced lipo protein and low density lipo proteins to human fibroblasts evidence for multiple receptor binding of apo e high density lipo protein c


, : Rate and equilibrium constants for binding of apo e high density lipo protein c a cholesterol induced lipo protein and low density lipo proteins to human fibroblasts evidence for multiple receptor binding of apo e high density lipo protein c. Proceedings of the National Academy of Sciences of the United States of America 76(5): 2311-2315

Competitive binding assays showed that a cholesterol-induced canine lipoprotein containing only the E apoprotein (apo-E HDLc) binds to the same cell surface receptors of human fibroblasts as human low density lipoproteins (LDL). The apo-E HDLc have a much greater binding activity than LDL. Equilibrium and kinetic binding studies were conducted at 4.degree. C to determine the mechanism for the enhanced receptor binding activity. Apparently the binding of both LDL and apo-E HDLc is a simple bimolecular receptor interaction; no heterogeneity of binding sites or cooperative effects among the receptor sites were seen. Equilibrium dissociation constants determined by Scatchard analysis of the equilibrium binding data for apo-E HDLc (Kd = 0.12 .times. 10-9 M) and LDL (Kd = 2.8 .times. 10-9 M) revealed a 23-fold greater affinity of HDLc for the receptors. Association and dissociation rate constants for the lipoprotein-receptor complex were determined from the time course of binding at various lipoprotein concentrations. The equilibrium dissociation constants calculated from these kinetic data confirmed that apo-E HDLc had a much higher affinity for the receptor than LDL. The kinetic studies showed that apo-E HDLc bound more rapidly than LDL with rates of association of 18.0 .times. 104 and 5.5 .times. 104 M-1 s-1, respectively. The rate of dissociation of the apo-E HDLc-receptor complex (1.7 .times. 10-5 s-1) was slower than that of the LDL receptor complex (6.3 .times. 10-5 s-1); 4 times (3.6 .+-. 0.4) as many LDL particles as HDLc particles were required for receptor saturation at maximal binding. Apparently each HDLc particle binds to multiple cell surface receptors at a ratio of 4:1 for LDL receptor binding.

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