Binding of Transferrin and Uptake of Iron by Rat Erythroid Cells in Vitro

Abstract

1. The binding of transferrin and the uptake of iron by rat bone-marrow-cell suspensions was investigated by the use of transferrin doubly labelled with ¹²⁵I and ⁵⁹Fe. 2. The pattern of transferrin binding was found to depend on the transferrin concentration in the incubation medium. At relatively low concentrations, binding of transferrin at 0–4 °C was lower than the binding at 37°C. At higher concentrations no difference could be observed between binding at 0–4°C and at 37°C. This phenomenon was explained in terms of a rapid non-specific adsorption of transferrin at 0–4°C, which takes place especially at higher transferrin concentrations, and a specific binding of transferrin at 37°C observed presumably at low concentrations. 3. The maximum number of specific transferrin-binding sites was found to be approximately 190 000 sites per rat reticulocyte and 330 000 sites per nucleated rat bone-marrow cell. The latter number corresponds to 500 000–700 000 sites per nucleated erythroid cell. 4. It was concluded that maturation of the erythroid cell is accompanied with a progressive loss of transferrin binding sites on the cell membrane. 5. When bone-marrow cells obtained after incubation with doubly-labelled transferrin were lysed with distilled water or with the detergent Nonidet P-40, differences in the subcellular distribution of the radioactivities could be observed. 6. It was concluded that the membrane fraction contains appreciable amounts of ⁵⁹Fe not bound to ¹²⁵I-labelled transferrin, which indicates that dissociation of the iron—transferrin complex is one of the earlier steps in the mechanism of iron uptake by erythroid cells.