LRP/CD91-mediated endocytosis of¹²⁵I-labeled Hx-heme and activated α₂-macroglobulin (α₂M^*) in COS-1 cells. (A) Time course of cell-associated radioactivity and degradation (measured as increase in TCA-soluble radioactivity) in cells incubated with ¹²⁵I-labeled Hx-heme. The inset shows Western blot of solubilized COS-1 cells with the monoclonal antibodies (refer to legend for Figure 1) against the LRP/CD91 subunits. (B) Time course of uptake and degradation of ¹²⁵I-labeled Hx-heme in the presence of the lysosomal inhibitors chloroquine and leupeptin (both 300 μM). (C-D) Similar experiments as in panels A and B using receptor-binding (methylamine-activated) ¹²⁵I-α₂-macroglobulin (α₂M^*) as radioligand instead. All values, which represent the measured radioactivity in percentage of the added radioactivity, are the mean ± 1 SD of triplicate determinations. Each well containing 300 μL medium was incubated with 3000 cpm ¹²⁵I-Hx-heme (∼0.5 ng).