RESEARCH FOR METAL-BINDING ABILITY AND SELECTIVITY OF VANADIUM-BINIDNG PROTEIN FROM ASCIDIANS

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Tunicates living in sea accumulate vanadium(V) ion from sea water and stores in their blood cells as vanadium(III) ion. However, this unique mechanism for vanadium accumulation and reduction has not been explored. In this project, we have obtained the basic knowledge concerning the vanadium in tunicates blood cells in order to explore the mechanism of accumulation and reduction of vanadium, using vanadium-binding protein and its small molecular model compounds.1.Reduction of vanadium(V) to vanadium(IV) by NADPH : It has been suggested that NADPH might be involved in the reduction of vanadium(V) by tunicates. We have already reported NADPH can reduce vanadium(V) to vanadium(IV) in the existence of edta. In this project, we have studied whether amino acids and peptide can act as a promoter of the reduction of vanadium(V) by NADPH. We have found that amino acids and peptides that can bind to vanadium as a tridentate ligand can promote the reduction while those that can bind to vanadium only as a bidentate fashion can not.2.Reduction of vanadium(IV) to vanadium(III) by thiol : We have shown that cystein methyl ester can promote the reduction of vanadium(IV) to vanadium(III) in the presence of edta. We have found that nta, glycylhistidine, and glycylaspartic acid can also promote the reduction of vanadium(IV) though partly. Lysine residue, which is rich in vanadium-binding protein, does not seem to relate to the reduction of vandium(V) and (IV).3.Oxidation of vanadium-binding protein by vanadium(III) : In order to examine whether the dithio bond can be reduced by vanadium(III), we examined the reaction between vanadium(III) and cystine or glutathione (oxidiazed form) as a model compound of vanadium-binding protein. We found that the dithio bond in the model compounds was cleaved by vanadium(III). The reduction of vanadium-binding protein by vanadium(III) is now under investigation.