The interaction of bovine transferrin and monoferric transferrin fragments with rabbit reticulocytes

1. The mechanism of interaction of transferrin with reticulocytes has been investigated using monoferric fragments derived by proteolysis from bovine Fe2-transferrin. 2. Rabbit reticulocytes readily took up iron from bovine transferrin, but only slight uptake occurred from the C-terminal fragment (S), and almost none from the N-terminal fragment (F). 3. The degree of binding of transferrin and fragments to the cells was in the order transferrin greater than fragment F greater than fragment S. 4. Binding of transferrin and fragment S, but not of fragment F, was reduced when incubation was performed at 4 degrees C instead of 37 degrees C, and all iron uptake was abolisehd. 5. Preincubation of reticulocytes with fragment S, but not with fragment F, somewhat reduced subsequent iron uptake from transferrin. 6. The presence of bovine serum albumin (40 mg/ml) in the incubation buffer inhibited iron uptake, but iron uptake nevertheless occurred from transferrin in bovine serum. 7. No differences were detected in the rate of 59Fe uptake from transferrin labelled asymmetrically by sequential additions of 59Fe and 56Fe to apotransferrin. 8. It is concluded that both halves of the transferrin molecule are involved, perhaps in different ways, in the interaction of transferrin with reticulocytes, and that rabbit reticulocytes do not take up iron preferentially from one of the binding sites of bovine transferrin.