Characterization of TFPIc23 binding to the VLDL receptor. (A) VLDL receptor-coated plates were incubated with ¹²⁵I-labeled TFPIc23 peptide plus increasing concentrations of unlabeled TFPIc23. Samples were incubated overnight at 4°C and washed, and bound TFPIc23 was measured in a γ counter. Each point is the mean of 3 wells ± SEM. (B) VLDL receptor was immobilized in microtiter plates, and the binding of RAP was assessed in the presence of increasing concentrations of the TFPIc23 peptide (○). After rinsing, bound RAP was quantified by ELISA using a RAP-specific rabbit polyclonal antibody. Control binding of RAP to BSA-coated wells was negative (•). (C) VLDL receptor was immobilized in microtiter plates, and the binding of ¹²⁵I-RAP was assessed in the presence of increasing concentrations of either TFPIc23 (•) or the hemipeptides TFPIc23A (○) and TFPIc23B (▾). RAP binding was quantified by using a γ counter.