Analysis of the iron-binding sites of transferrin by isoelectric focussing

1. Human transferrin was labelled with ferric nitrilotriacetate (FeNTA) at one of its two metal binding sites by variation of the pH. 2. Four transferrin forms, transferrin, transferrin(Fe) (A-site), transferrin(Fe) (B-site) and transferrin(2Fe) could be separated on flat bed gels by isoelectric focussing. 3. Incubation time, temperature and medium play an important role in the specificity of the binding of Fe. In NTA-pH-buffer, at 0.5 Fe-saturation, the A-site was preferentially labelled at pH 7--8, the B-site at a pH 8--9. 4. Under physiological conditions iron from the B-site has the tendency to move to the A-site.