A Comparison of the Behavior of ¹¹¹In and ⁵⁹Fe-labeled Transferrin on Incubation With Human and Rat Reticulocytes

The uptake by human and rat reticulocytes of ¹¹¹In and ⁵⁹Fe 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 ¹¹¹In from human and rat serum was 30% and 12% that of the ⁵⁹Fe after a 30-min 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 min with either ⁵⁹Fe or ¹¹¹In bound to serum were reincubated in unlabeled serum. Up to 85% of the ¹¹¹In label and less than 10% of the ⁵⁹Fe on the reticulocytes were released on reincubation, indicating that, in contrast to ⁵⁹Fe, the majority of the ¹¹¹In label remained membrane bound. Specific binding of unlabeled In transferrin was demonstrated by its inhibitory effect on ⁵⁹Fe-transferrin uptake by rat reticulocytes. Both In- and Fe-transferrin were found to have similar binding affinities for the receptor sites. The ¹¹¹In remaining in lysates obtained from washed reticulocytes after incubation with ¹¹¹In-labeled serum and reincubation in unlabeled serum did not appear to be associated with either the hemoglobin or heme molecules.