Vanadium (V) is an essential metal for mammals and has different valence states. In blood, V is bound to serum transferrin (P), a glycoprotein which has two metal-binding sites, and carbonate is generally required for the binding. In this study, the binding patterns of V(III), V(IV), and V(V) to human serum Tf (hTf) were analyzed using an HPLC system equipped with an anion-exchange column and directly connected to a high-resolution inductively coupled plasma-mass spectrometer for metal detection (I IV). In affinity to hTf, the three ions were ranked V(III) > V(IV) > V(V) in the presence of bicarbonate and V(III) congruent to V(IV) > V(V) in the absence. Intermediates in the "open forms" binding to the respective sites were detected at the initial stage. V(IV) and V(V) were bound to the N-lobe site in the "closed form" and "open form," respectively. In the absence of bicarbonate, V ions with respective valence states were bound to hTf in the "open form." In terms of binding to hTf, tri-valent V was most favorable in the presence of bicarbonate. (C) 2002 Elsevier Science (USA). All rights reserved.