Ascidian vanadocyte

Figure 17.16 Model of the pathway for reduction and accumulation of vanadium in ascidian vanadocytes. (From Crans et al., 2004. Copyright (2004) American Chemical Society.)...

FIGURE 10.1 Ascidian vanadocyte. Modified from Reference 26. 

The sea squirts or tunicates are fascinating marine creatures, their name being derived from the tunic made of cellulosic material that surrounds the body of the animal. In 1911, Henze discovered vanadium in the blood of Phallusia mammillata C.343 He later found the same with other ascidians (a class of tunicates). In vanadium-accumulating species, most vanadium is located in the vacuoles—vanadophores—of certain types of blood cells—the vanadocytes. The concentration in the vanadophore can be as high as 1M and this value must be compared with concentrations of the order of 2 x 10-8 M for vanadium in sea water.344 Kustin et al. have reviewed the work done to understand the efficient accumulation and the possible biological roles of the metal.345... 

The ascidians or tunicates (sea squirts) accumulate vanadium from seawater (about 5x 10-8 mol dm-3) to a level of about 1 mol dm-3 and store it in a dilute solution of sulfuric acid (pH<2) in blood cells called vanadocytes. The tunicates thus concentrate vanadium several million-fold. 079 NMR, ESR and EXAFS determinations on whole vanadocyte cells of Ascidia ceratodes and Ascidia nigra indicate that the vanadium is present mainly as aquated V111 probably complexed with sulfate. Some vanadyl ion (5-10%) is also present.1080 1081... 

Ueki, T., N. Yamaguchi, and H. Michibata. 2003. Chloride channel in vanadocytes of a vanadium-rich ascidian Ascidia sydneiensis samea. Comp. Biochem. Physiol. B Biochem. Mol. Biolog. 136 91-98. 

Yamaguchi, N., K. Kamino, T. Ueki, and H. Michibata. 2004. Expressed sequence tag analysis of vanadocytes in a vanadium-rich ascidian, Ascidia sydneiensis samea. Marine Biotech. 6 165-174. 

Therefore I submit that vanadocytes enable ascidians to sustain periods of oxygen deprivation (anoxia), in contradistinction to proposals suggesting that they serve as oxygen carriers. ... 

It appears to be well established by now that the reductant responsible for the conversion of vanadate(V) to oxovanadium(IV) is nicotine adenine dinucleotide phosphate (NADPH), produced in the pentose phosphate pathway.PI Further reduction to in the vacuoles of the vanadocytes of Phlebobranchia ascidians remains elusive. Several reductants have been proposed and shown to be effective in the in vitro and/or in vivo reduction of to V and/or to including tunichromes. Selected investigations of the related redox chemistry of vanadium can be briefly summarised as follows ... 

Michibata H, Uyama T, Ueki T and Kanamoei K (2002) Vanadocytes, cells hold the key to resolving the highly selective accumulation and reduction of vanadium in ascidians. Microsc Res Tech 56 421-434. 

Some ascidians accumulate vanadium (vanadocytes in blood cells) in concentrations that exceed that in sea water some 4... 

Other studies investigated marine animals such as ascidia (subtype of Ttini-cata ) and holothurians (type Echinodermata ). Peterson [8] is only one of the many authors that have pointed out the high vanadium concentrations common in ascidians [7]. These animals contain 10,000 times the vanadium that is present in the water in which they live [5]. In ascidians, special green blood cells called vanadocytes concentrate the element. Vanadium in these cells is in the trivalent state, is complexed to pyrrole rings, and is associated with unusually high concentrations of sulfuric acid [4]. In other ascidians, vanadium is concentrated in the blood plasma (hemovanadium) rather than in individual cells. Although the vana-... 

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