Some ascidians sequester vanadium and other metal ions that are bound and transported in higher organisms by transferrin. The ascidian Ciona intestinalis has a monolobal transferrin (nicatransferrin) in its plasma. The binding of vanadium(V) to nicatransferrin was investigated by using isothermal titration calorimetry and UV-vis spectroscopy, in the presence and absence of NaHCO(3), and was compared with human serum transferrin. Nicatransferrin and serum transferrin bind V(V) with similar strengths [K = (2.0 +/- 0.6) x 10(5) M(-1) for nicatransferrin]; however, nicatransferrin requires a synergistic anion for V(V) binding, whereas serum transferrin does not. Spectroscopy supports a different kind of coordination site.