Abstract
Fits to the vanadium K-edge X-ray absorption spectra (XAS) of five whole blood cell samples from the tunicate Phallusia nigra revealed unprecedented forms of intracellular vanadium. Endogenous vanadium was divided between the V(III) ion (74.2±5.1% of total V) and the vanadyl ion [V(IV)O]2+ (25.2±5.4% of total V). The V(III) fraction included both [V(H2O)6]3+ (36.7±5.5%) modeled as VCl3 in 1 M HCl, and three previously unprecedented chelated V(III) forms (37.5±4.6%). Two of these could be represented by the model ligand environments V(acetylacetonate)3 (17.9±3.2%) and K3V(catecholate)3 (13.1±4.7%), implying DOPA-like complexation. The third chelated form was represented by the 7-coordinate N2O5 complex Na[V(edta)(H2O)] (8.0±1.8%). This coordination array, suggestive of a novel mononuclear V(III) protein site, contributed only to fits to samples 1, 2, 3 and 5, which were prepared in the presence of DTT. Endogenous V(IV) (25.2±5.4%) was principally modeled as VOCl2 in 1 M HCl. EPR spectra (averages: A‖=(1.842±0.006)×10−2 cm−1; A⊥=(0.718±0.007)×10−2 cm−1; g‖=1.936±0.002; g⊥=1.990±0.001) confirmed the predominance of the aquated vanadyl ion. Blood cell sample five uniquely required the XAS spectrum of VOSO4 in 0.1 M H2SO4 solution (13.0%) and of [OV(V)(pivalate)3] (3.1%) to successfully fit the XAS pre-edge energy region. This endogenous V(V) signal is also unprecedented. These results are compared with those of analogous fits to the blood cells of Ascidia ceratodes and may support assignment of P. nigra to a different genus.