Figure 7

¹²⁵I-α ₂ M internalization, but not ¹²⁵I-apoE-VLDL, is inhibited by K⁺ depletion of cells. GM00701/Syn-1-HA cells were treated with K⁺-depleted buffers (open symbols), or K⁺-containing serum-free media (closed symbols), as described in Methods. Cells were incubated with either ¹²⁵I-apoE-VLDL (2 μg/ml) (circles) or ¹²⁵I-α₂M (0.5 μg/ml) (triangles) for 20 min at 37°C. Specificity of uptake was determined by a parallel incubation with either unlabeled apoE-VLDL (100 μg/ml) or unlabeled α₂M (5 μg/ml), respectively. After the 37°C incubation, cells were chilled to 4°C and acid stripped with 50 mM glycine, pH 3, 100 mM NaCl for 5 min to remove surface bound ligand (that which was not internalized). Internalized ligand was quantitated by solubilizing cells with 0.1 N NaOH and scintillation counting.