Mobilization of iron from the plasma membrane of the murine reticulocyte. The role of ferritin.

1. Plasma membranes prepared by pre-incumbation of mouse reticulocytes with 125I, 59Fe-labeled murine transferrin were able to release 59Fe in preference to 125I when incubated in the presence of murine reticulocyte cytosol, demonstrating that the latter mobilized iron which had been dissociated from transferrin. 2. 59Fe in cytosol was associated with at least two components in addition to hemoglobin, a high molecular weight component, identified as ferritin by specific immunoprecipitation, and an as yet unidentified, low molecular weight component of approx 17 00. 3. Ferritin itself, in the absence of added cytosol, was abloe to mobilize 59Fe from s9Fe-labeled reticulocyte plasma membranes. 4. Lysates of reticulocytes synthesized 59Fe-labeled heme when incubated with 59Fe-labeled ferritin. 5. These findings reflect a pathway of iron uptake and incorporation into heme in which ferritin plays an active role.