Binding of xylenol orange to transferrin. Demonstration of metal-anion linkage.

The iron transport protein, transferrin, binds two metal ions and, concomitantly, two carboxylate anions. The metal ion indicators, xylenol orange and semi-xylenol orange are carboxylate anions which exhibit a characteristic visible spectrum when attached to a metal. We prepared the ternary complexes VO2+-transferrin-xylenol orange and VO2+-transferrin-semi-xylenol orange. The EPR spectra show that the vanadyl ion is attached to the protein and the visible spectra show that the xylenol orange or semi-xylenol orange is attached to the metal. The implication is that in other metal-transferrin-anion complexes, the anion is directly attached to the metal.