Changes in the uptake of transferrin-free and transferrin-bound iron during reticulocyte maturation in vivo and in vitro.

The uptake of non-transferrin-bound iron, Fe(II), transferrin-bound iron, Tf-Fe and transferrin was studied in reticulocytes from anaemic rabbits during maturation and then synchronized regeneration in vivo (following injection of actinomycin D) and while maturing during in vitro incubation. The uptake of Fe(II) and Tf-Fe decreased in parallel with each other and with the reticulocyte count and transferrin uptake during maturation in vivo and in vitro. Only during the early phase of reticulocyte regeneration in vivo was there a significant difference between the rates of Fe(II) and Tf-Fe uptake. These results suggest that a membrane carrier for iron and the transferrin receptor are lost at the same rate during reticulocyte maturation, possibly because they are associated with each other in the cell membrane. During reticulocyte maturation the rate of Fe(II) uptake into heme declined more rapidly than uptake into the total cellular cytosol. The loss of transferrin receptors and the uptake of iron from transferrin during reticulocyte maturation was not associated with a change in the affinity of the receptors for transferrin, in the relative distribution of the receptors between the outer cell membrane and intracellular sites or in the ability of the transferrin molecule to donate two iron atoms to the cell with each intracellular cycle, but the average duration of the cycle increased.