Binding of apoB-containing lipoproteins from unfractionated human blood sera to immobilized LDL receptor

Binding of apoB-containing lipoproteins from unfractionated human blood sera to the immobilized bovine receptor of low density lipoproteins (LDL receptor) was studied. Peroxidase-labeled anti-human apoB antibodies were used to evaluate the lipoprotein binding. The equilibrium dissociation constant (Kd) of the interaction between apoB-containing lipoproteins from unfractionated human sera from healthy donors and the immobilized LDL receptor varied from 1 to 20 microg apoB/ml. To describe the binding of lipoproteins to the LDL receptor, a parameter of relative binding affinity (RBA) was used. RBA is inversely related to value of Kd and equal to unity for the standard serum. The RBA values for the binding of apoB-containing lipoproteins from unfractionated sera to the immobilized LDL receptor were found to correlate with the RBA values for the binding of isolated VLDL (r = 0.76, p < 0.001) and fail to correlate with the RBA values for the binding of isolated LDL. The RBA values for the binding of apoB-containing lipoproteins from unfractionated sera correlated with the RBA values for the binding of apoE-containing lipoproteins from unfractionated sera (r = 0.92, p < 0.001) and with values of triglyceride concentration in the sera (r = 0.93, p < 0.001). The RBA values for the binding of apoB-containing lipoproteins from sera of patients with FDB whose LDL were unable to bind to the LDL receptor did not significantly differ from the RBA values for the normal sera. However, the removal of VLDL from the normal sera significantly decreased the RBA values for the binding of apoB-containing lipoproteins from unfractionated sera. The results indicate that the different binding of apoB-containing lipoproteins to the immobilized LDL receptor mainly depended on the different binding of VLDL and not of LDL.