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

Binding of apoB-containing lipoproteins from unfractionated human blood ser a to the immobilized bovine receptor of low density lipoproteins (LDL recep tor) was studied. Peroxidase-labeled anti-human apoB antibodies were used t o evaluate the lipoprotein binding. The equilibrium dissociation constant ( K-d) of the interaction between apoB-containing lipoproteins from unfractio nated human sera from healthy donors and the immobilized LDL receptor varie d from 1 to 20 mu g 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 K-d and equal to unity for the standard s erum. The RBA values for the binding of apoB-containing lipoproteins from u nfractionated sera to the immobilized LDL receptor were found to correlate with the RB A values far 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 unfrac tionated sera correlated with the RBA values for the binding of apoE-contai ning lipoproteins from unfractionated sera (r = 0.92, p < 0.001) and with v alues 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 pat ients with FDB whose LDL were unable to bind to the LDL receptor did not si gnificantly differ from the RBA values for the normal sera. However, the re moval of VLDL from the normal sera significantly decreased the RBA values f or the binding of apoB-containing lipoproteins from unfractionated sera. Th e results indicate that the different binding of apoB-containing lipoprotei ns to the immobilized LDL receptor mainly depended on the different binding of VLDL and not of LDL.