Vanadium accumulation in ascidians: A system overview

• Ascidians accumulate extremely high levels of vanadium in their blood cells.
• The metal selectivity of this system is due to a combination of vanadium-binding proteins, vanadium transporters, and vanadium reductase.
• The high selectivity of Vanabin2 reductase may be the key factor for metal ion selectivity.
• The physiological function of accumulated vanadium ions in a reduced form, VIII, is presumably exerted through redox-coupled reactions.

Several families of ascidians accumulate extremely high levels of vanadium in their blood cells. The concentration of vanadium has been determined in each species; the highest concentration, found in Ascidia gemmata, reaches 350 mM, corresponding to 107 times that of sea water. How and why ascidians accumulate vanadium in a highly selective manner and at such extremely high levels have yet to be determined. To address these questions, our research group sought to identify the genes and proteins responsible for the accumulation and reduction of vanadium in vanadocytes, a type of blood cell, as well as the process of vanadium transport from sea water to blood cells through the branchial sac, intestine, and blood plasma. Here, we review the accumulation steps as a system, especially those related to the concentration and chemical species of vanadium at each step. A comprehensive analysis on each organ has already revealed several categories of protein families, such as vanadium-binding proteins and vanadium transporters. Herein, we also discuss the mechanisms by which ascidians selectively accumulate vanadium ions from a biochemical viewpoint.