The Metabolism of Transferrin-bound ¹¹¹In and ⁵⁹Fe in the Rat

The metabolism of transferrin-bound indium and iron were compared by in vivo studies in the rat. Rats were injected with serum transferrin labeled with ¹¹¹In and ⁵⁹Fe, and, at intervals ranging from 20 min to 5 days after injection, they were killed. They were perfused through the portal vein with 200 ml of 0.9% saline, and the residual radioactivity, expressed as a percentage of the injected dose, was measured in liver, spleen, kidney, muscle, washed red cells, red marrow, and femur. At 5 days, 76% of the injected ⁵⁹Fe was recovered in the red cell mass; only 1%-2% of ¹¹¹In could then be recovered. Uptake and release of the ¹¹¹In label by the femur was markedly less than that of the ⁵⁹Fe. Whereas 85% of the injected ⁵⁹Fe could be recovered from the circulating red cells, liver, and spleen, only about 15% of the injected ¹¹¹In could be so recovered. Approximately 35% of the injected ¹¹¹In was excreted, and 43% was recoverable from the carcass. The subcellular distribution of the two isotopes in the liver at timed intervals following intravenous injection was studied. While 35% of the ⁵⁹Fe activity in the homogenate was associated with ferritin, only 4% of the ¹¹¹In could be so identified. The results indicate a significant difference between the metabolism of ¹¹¹In and ⁵⁹Fe in the rat and make it unlikely that the metabolism of ^In in man bears much similarity to that of iron.