Qualitative binding of lactoferrin (), PAI-1 (▩), and RAP (□) to LRP fragments. The ability of various LRP fragments to bind iodinated protein ligands at a single concentration (25 nmol/L) was investigated by direct binding assay. ¹²⁵I-RAP,¹²⁵I-lactoferrin, and ¹²⁵I-PAI-1 were bound to 5 pmol of various LRP fragments in the presence of 2 mmol/L CaCl₂. BSA was used as a nonreceptor control. Nonspecific binding was measured in parallel wells without calcium and in the presence of 40 mmol/L EDTA. Fragment E4-C10 was used as the 100% binding control and had the following cpm: ¹²⁵I-RAP = 31,707 ± 1433; ¹²⁵I-lactoferrin = 14,072 ± 255; and¹²⁵I-PAI-1 = 4,098 ± 60. Triplicate points in duplicate experiments were used to calculate a mean and SD for each receptor.