Iron-donating properties of transferrin.

The transferrin molecule has two specific metal-binding sites, each of which may provide iron for the biosynthesis of hemoglobin by reticulocytes. Diferric human transferrin was shown to be a better iron donor, per iron atom, for rabbit reticulocytes, than was monoferric transferrin obtained by isoelectric focusing. The difference in binding of 125I-labeled monoferric and differic transferrin to reticulocytes may be sufficient to account for the difference in iron uptake. In contrast, diferric and monoferric rabbit transferrin both donated iron to reticulocytes at the same rate, per iron atom. In an experiment using 55Fe/59Fe doubly labeled transferrin, one iron binding site of human transferrin was a better iron donor than the other. In rabbit transferrin, the two sites appeared to function equivalently. Care was taken in these experiments to demonstrate that labeled iron added to dilute solutions of transferrin was indeed specifically bound to the protein. A liquid scintillation counting procedure, simpler than existing methods, was developed to quantitate 55Fe and 59Fe in blood.