Reciprocal competition of LRP ligands on binding to C5-C7 or E4-C10. Binding of 10 nmol/L ¹²⁵I-lactoferrin (A and B) was competed with RAP (○) and active PAI-1 (□); binding of 2 nmol/L¹²⁵I-RAP (C and D) was competed with lactoferrin (•) and active PAI-1 (□). Triplicate experiments were used to calculate a mean and SD for each data point. In each experiment, the concentration of the competing unlabeled ligand ranged from 0.01 pmol/L to 10 μmol/L. Nonspecific binding was measured in parallel wells without calcium and in the presence of 40 mmol/L EDTA. The IC₅₀value for each competing ligand was determined by using a single-site competition model and is shown in Table 3.