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

Abstract

Abstract 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.