Binding sites of iron transferrin on rat reticulocytes. Inhibition by specific antibodies.

1. In the process of iron uptake by precursors of the erythrocytes probably more than one membrane component is involved; besides the specific transferrin receptor, another membrane component with a high iron activity after incubation with 59Fe can be isolated. 2. A striking resemblance exists between rat and human reticulocyte components which are involved in the process of iron uptake. 3. Incubation of reticulocytes with Fab fragments of an antibody against the membrane receptor for transferrin causes a concentration-dependent decrease in transferrin binding and iron uptake. 4. The membrane receptor complex isolated is still heterogeneous; analytical ultracentrifugation studies suggest a molecular weight lower than 230,000. 5. Intact immature red cells are necessary for specific binding of transferrin with the receptor followed by iron uptake. This is the only mechanism for iron uptake. 6. Immunofluorescence studies showed that the receptor for transferrin is localized at the outside of the cell membrane.