Binding of vanadium to human serum transferrin - voltammetric and spectrometric studies

Previous studies generally agree that in the blood serum vanadium is transported mainly by human serum transferrin (hTF). In this work through the combined use of electrochemical techniques, matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry and small-angle X-ray scattering (SAXS) data it is confirmed that both V^IV and V^V bind to apo-hTF and holo-hTF. The electrochemical behavior of solutions containing vanadate(V) solutions at pH = 7.0, analyzed by using two different voltammetric techniques, with different time windows, at a mercury electrode, Differential Pulse Polarography (DPP) and Cyclic Voltammetry (CV), is consistent with a stepwise reduction of V^V → V^IV and V^IV → V^II. Globally the voltammetric data are consistent with the formation of 2:1 complexes in the case of the system V^V-apo-hTF and both 1:1 and 2:1 complexes in the case of V^V-holo-hTF; the corresponding conditional formation constants were estimated. MALDI-TOF mass spectrometric data carried out with samples of V^IVOSO₄ and apo-hTF and of NH₄V^VO₃ with both apo-hTF and holo-hTF with V:hTF ratios of 3:1 are consistent with the binding of vanadium to the proteins. Additionally the SAXS data suggest that both V^IVOSO₄ and NaV^VO₃ can effectively interact with human apo-transferrin, but for holo-hTF no clear evidence was obtained supporting the existence or the absence of protein-ligand interactions. This latter data suggest that the conformation of holo-hTF does not change in the presence of either V^IVOSO₄ or NH₄V^VO₃. Therefore, it is anticipated that V^IV or V^V bound to holo-hTF may be efficiently up-taken by the cells through receptor-mediated endocytosis of hTF.