A0Comparison of the Behavior of “in and Fe-labeled Transferrin on Incubation With Human and Rat Reticulocytes

The uptake by human and rat reticulocytes of and “Fe bound to transferrin has been studied. The results indicate a significant difference between the behavior of the two isotopes in both human and rat incubation mixtures. Reticulocyte uptake of “In from human and rat serum was 30% and 12% that of the “Fe after a 30-mm incubation. The process was temperature dependent, inhibited by sodium arsenite, and related to the reticulocyte percentage of the cells in the reaction mixture. Washed reticulocytes, previously incubated for 30 mm with either Fe or “In bound to serum were reincubated in unlabeled serum. Up to 85% of the 11’ln label and less than 10% of the Fe on the reticulocytes were released on reincubation, indicating that, in contrast to Fe, the majority of the “In label remained membrane bound. Specific binding of unlabeled In transferrin was demonstrated by its inhibitory effect on Fe transferrin uptake by rat reticulocytes. Both Inand Fe-transferrin were found to have similar binding affinities for the receptor sites. The remaining in lysates obtained from washed reticulocytes after incubation with “In-labeled serum and reincubation in unlabeled serum did not appear to be associated with either the hemoglobin or heme molecules.