Vanabins (also known as vanadium-associated proteins or vanadium chromagen) are a class of metalloproteins containing Vanadium. Found only in the Blood of some ascidians and tunicates (sea squirts), these organisms are able to concentrate vanadium to a level more than 100 times higher than in the surrounding seawater. Vanabin proteins seem to be involved in collecting and accumulating this metal ion. At present there is no conclusive understanding of why these organisms collect vanadium, and it remains a biological mystery.
The highest concentration of vanadium found so far, 350 mM, was found in the blood cells of Ascidia gemmata belonging to the suborder Phlebobranchia. This concentration is 10,000,000 times higher than that in seawater.
Vanabins accumulate vanadium in the blood cells and reduce V3+ and V4+ from V5+ with the use of NADPH. Vanabins also transport the reduced vanadium ions into the vacuoles of vanadocytes (vanadium-containing blood cells). The vacuoles are kept at a very acidic pH of 1.9, made possible by pumping hydrogen ions into the vacuoles by use of energy intensive H+-ATPase. All the use of NADPH and ATP to collect and maintain the vanadium is extremely energy intensive, unusual for oxygen transporting proteins.
Other oxygen transporting proteins have a very low Dissociation constant with their metal prosthetic group and bind these groups tightly. Vanabins on the other hand have a moderate dissociation constant and do not tightly bind vanadium. Most importantly, because of this moderate dissociation constant, vanadium is usually found free-floating and separated from any proteins inside the vacuoles. This is completely different from other oxygen transporting proteins.
The use of vanabins and vanadium for oxygen transport in ascidians and tunicates is doubtful. Another hypothesized reason for these organisms collecting vanadium is to make themselves toxic to predators, parasites and microorganisms.