The two stages in the uptake of transferrin by rabbit reticulocytes were investigated using radioiodine-labeled rabbit transferrin and albumin. The first stage of rapid, temperature-insensitive uptake of transferrin was similar to albumin uptake: uptake of both proteins increased linearly with increasing Protein concentration of the incubation medium up to at least 60 mg/ml, was maximal at low ionic strength and pH, and increased in the presence of basic polyamino acids. Transferrin uptake was in part dependent on the reticulocyte concentration of the blood, but albumin uptake was independent of reticulocyte concentration.
The second slower, temperature-sensitive stage of transferrin uptake was linearly related to reticulocyte concentration, and was not found with albumin, al-macroglobulin or 7-globulin. Transferrin uptake was optimal at physiological pH and ionic strength and was unaffected by basic polyamino acids. When the transferrin concentration was raised, uptake increased to reach a maximum at a concentration of 15 mg/ml.
It was concluded that the first stage of transferrin uptake was in part or wholly due to non-specific adsorption of transferrin to erythrocytes, while the second stage of uptake was specific for transferrin and reticulocytes and depended upon normal function of the cells.
The mammalian reticulocyte retains the ability to take up iron from the plasma ironbinding protein, transferrin, and to incorporate it into newly synthesized hemoglobin. This capacity is lost when the reticulocyte matures (Walsh et al., '49). The first step in iron uptake is the binding of transferrin to the cell (Jandl and Katz, '63; Morgan and Laurell, '63). This occurs in two stages, rapid adsorption followed by slower progressive uptake (Morgan, '64a; Baker and Morgan, '69a). The actual mechanism of the interaction between transferrin and reticulocytes is poorly understood. The purpose of this work was to investigate this problem from the point of view of the specificity of the reaction for transferrin and for reticulocytes and the nature of the forces involved in the uptake of transferrin by the cell. The experiments were performed with purified transferrin and other proteins from rabbit serum labeled with radioactive iodine, and rabbit reticulocytes.