Some Specific Interactions of Small Ions with Transferrin

Abstract

The fully iron saturated complex of transferrin was reconstituted from the apoprotein in the absence of bicarbonate but in the presence of other small anions. Bicarbonate was not found to be essential for the specific iron binding of the protein. The absorbance of the various complexes at 465 mμ was found to vary for each anion used to replace bicarbonate The simulataneous uptake of ferric iron and bicarbonate by apotransferrin was studied by equilibrium dialysis. The mean number of ferric ions bound per protein molecule was two. One bicarbonate ion was associated with each of the specifically bound ferric ions. Competitive binding experiments, in which other small anions displaced the bound bicarbonate, showed that oxalate was the strongest competitor for union with the bound iron. An oxalate ferric transferrin complex was prepared with the same stoichiometry as the bicarbonate complex. The optical rotatory dispersion properties of these complexes were compared in the visible-near ultraviolet region. The displacement of bicarbonate from the bicarbonate-ferric-transferrin complex, by oxalate, and the converse reaction, using oxalate-ferric-transferrin, were studied. Equilibrium equations are proposed for these reactions and the equilibrium constants calculated. The anomalous results obtained are discussed in terms of the mode of binding of bicarbonate and oxalate.