Pulse-chase of COS-1 cells incubated with¹²⁵I-labeled Hx-heme and α₂-macroglobulin. The figure shows the time course of the appearance of TCA-precipitable and soluble radioactivity in the medium after pulse with ¹²⁵I-Hx-heme (A) and methylamine-activated ¹²⁵I-α₂-macroglobulin (α₂M^*) for 60 minutes at 4°C followed by wash of the cells and incubation at 37°C (B). All values, which represent the measured radioactivity in percentage of the initial cell-associated radioactivity, are the mean ± 1 SD of triplicate determinations.