A novel vanadium reductase, Vanabin2, forms a possible cascade involved in electron transfer

The unusual ascidian ability to accumulate high levels of vanadium ions at concentrations of up to 350 mM, a 107-fold increase over that found in seawater, has been attracting interdisciplinary attention for a century. Accumulated VV is finally reduced to VIII via VIV in ascidian vanadocytes. Reducing agents must therefore participate in the reduction. Previously, we identified a vanadium-binding protein, Vanabin2, in which all 18 cysteines form nine disulfide bonds. Here, we report that Vanabin2 is a novel vanadium reductase because partial cleavage of its disulfide bonds results in the reduction of VV to VIV. We propose that Vanabin2 forms a possible electron transfer cascade from the electron donor, NADPH, via glutathione reductase, glutathione, and Vanabin2 to the acceptor, and vanadium ions conjugated through thiol–disulfide exchange reactions.