Ferritin and iron uptake by reticulocytes

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Abstract
The uptake of liver ferritin labeled with 125I or 59Fe by guinea-pig reticulocytes was studied to investigate the characteristics of the uptake process, compare it with transferrin uptake and determine whether ferritin-Fe is utilized by the cells in heme synthesis. Guinea-pig reticulocytes, but not mature erythrocytes, apparently take up liver ferritin by a saturable, time- and temperature-dependent process. Up to 70% of the Fe taken up by the cells was utilized in heme synthesis and competed directly with Fe derived from transferrin. Scatchard analysis of the binding parameters indicated that 30-130 .times. 103 ferritin molecules were bound/cell to high affinity specific membrane receptors (Ka: 1.77 .times. 107 M-1). In contrast, rat took up much less ferritin than guinea-pig reticulocytes and the process was entirely non-specific. Release experiments with guinea-pig reticulocytes at 37.degree. C showed that a maximum of about 70% of the cell-associated 125I-ferritin was released from the cells of which up to 15% was trichloroacetic acid-soluble. Ferritin uptake by guinea-pig reticulocytes apparently involves receptor-mediated endocytosis. The endocytotic vesicle fuses with a lysosome, Fe is removed from the protein and enters a cytosolic pool in which it competes directly with transferrin-derived Fe to provide Fe for mitochondrial heme synthesis. Some of the ferritin is catabolized and the rest is returned to the extracellular medium during membrane recycling.

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