Vanadium speciation in healthy human serum: Binding patterns of vanadium to transferrin studied by HPLC/HIGH-resolution ICP-MS

Vanadium (V) is suggested to be essential for mammals. It has different valence states. In blood, V is bound to transferrin (Tf), a glycoprotein that has two metal-binding sites (C-lobe site and N-lobe site). In the present study, the binding patterns of V to serum Tf were analyzed by online combination of HPLC and high-resolution ICP-MS (HPLC/HR-ICP-MS) with an anion-exchange column. The levels of V-51, Fe-56 and S-32 were monitored simultaneously by HR-ICP-MS at a resolution of m/Delta m = 4,000. First, the binding of V ions to two lobes of apo-human serum Tf (hTf) in the presence and absence of bicarbonate was studied in three different valence states (V(III), V(IV) and V(V)). The three ions ranked V(III)> V(IV)> V(V) in affinity to hTf in the presence of bicarbonate. Secondly, a 1 ml portion of serum from a healthy person was directly subjected to column. V in human serum without any in vitro V spike was detected as VC-Tf. Since V(III) was most favorable in terms of the binding to hTf in the presence of bicarbonate and V bound to the C-lobe site of hTf was detected only in the case of V(III) among the three valence states of V, it was suggested that a part, at least, of V in the VC-Tf in healthy human serum may be present as V(III), in addition to the generally accepted V(IV).