Isolation, properties and structural studies on a compound from tunicate blood cells that may be involved in vanadium accumulation

Abstract

A novel compound for which the trivial name tunichrome was proposed, was isolated from V-rich blood cells of the tunicate A. nigra. Preliminary structural studies suggested MW of about 390, the presence of conjugated vinyl groups, and an acidic group, possibly carboxyl, with an apparent pKa of 3.0. Elements C, H, N and O comprised 98.4% of the sample weight, the number of atoms/mol of tunichrome being 14.1, 22.2, 1.5 and 10.6, respectively, which indicated some heterogeneity in the sample. Tunichrome readily reduced Fe(III) and V(V). In an initial fast step, 2 mol of V(V) were reduced, or 4 mol of Fe(III)-phenanthroline/mol of tunichrome; another 9 mol of Fe(III)-phenanthroline or Fe(III)-bipyridine were reduced in a slow reaction. The initial reaction was 1st-order with respect to tunichrome and Fe(III). Above pH 3.5, tunichrome is rapidly hydrolyzed, 13 mol of OH- being consumed/mol of tunichrome. Hydrolysis involves polymerization and loss of characteristic absorption peak at 325 nm. Tunichrome presence may be linked to V accumulation by blood cells. The mechanism involves vanadate entry via an anionic channel into blood cell vacuoles, where it was reduced to V(IV) or V(III), both of which being cationic, cannot escape the vacuole.