Effect of protease inhibitors on ferritin accumulation in cultured cells. (A) Effect of chymostatin. Chymostatin (200 μg/mL) was added at days 7 or 8 of phase II (EPO-dependent phase, see Materials and Methods), after 24 hours the same amount of chymostatin was added again. The cells were harvested at days 10 or 11 and the ferritin concentrations in the lysates were determined by ELISA. Results are expressed as mean ± SD of three individual experiments performed in duplicate. (B) The effect of chymostatin and leupeptin. Leupeptin (10 μg/mL) was added together with chymostatin (added as above).