Release of iron from endosomes is an early step in the transferrin cycle

Abstract

1. 1. Transferrin bound to K 562 cells at 4°C was internalized quickly on temperature shift to 37°C. 2. 2. Endosomes were isolated according to two different procedures. 3. 3. The endosome fraction was shown to be heterogenous and consisted of two vesicle populations, differing in density properties and iron content. 4. 4. Iron was partially released from endosomes to the supernatant after 3 and 5 min endocytosis. 5. 5. Isolated endosomes, still capable of internal acidification, did not release iron on incubation with ATP. 6. 6. However, endosomes did release iron on incubation with the iron chelator pyridoxal-isonicotinoyl hydrazone. 7. 7. Gelfiltration of solubilized endosomes demonstrated the presence of the transferrin-transferrin receptor complexes, free transferrin and free low molecular weight iron.