Dependence of inhibition of ¹²⁵I-RAP degradation on soluble E4-C10. Replicate monolayers of MEF 1 fibroblasts were incubated with serum-free DMEM containing¹²⁵I-RAP and different concentrations of LRP fragment E4-C10. After incubation at 37°C for 4 hours, the total amount of¹²⁵I-RAP degradation products secreted into the media was measured. The points are the mean of triplicates from one to three experiments and are normalized to the amount of degradation products released in the absence of E4-C10.