Comparative study of the iron-binding properties of human transferrins. II. Electron paramagnetic resonance of mixed metal complexes of human lactotransferrin.

Human lactotransferrin is able to bind two vanadyl(IV) ions in specific metal-binding sites. The EPR signals of the two vanadyl bound ions, however, appear as one. This result suggests that the environments of the binding sites of human lactotransferrin are similar. The binding activity is promoted to pH 4 using carbonate or bicarbonate as synergistic anion. This unusual stability of the anion-binding site, which is destroyed below pH 6 for other transferrins, can explain in part the great stability of the metallic complexes of human lactotransferrin. However, the different sensitivities of the two metal-binding sites towards protonation permit the preparation of mixed vanadyl(IV), iron(III) complexes with VO2+ bound either on the N-terminal (acid-labile or B site) or on the C-terminal (acid-stable or A site) site. Analysis of the spectra of such mixed complexes shows the presence of a third nonspecific VO2+-binding site termed A'. The nonspecific A' site seems to be located on the outer surface of the protein close to the C-terminal site.