Unprecedented forms of vanadium observed within the blood cells of Phallusia nigra using K-edge X-ray absorption spectroscopy

Fits to the vanadium K-edge X-ray absorption spectra (XAS) of five whole bl ood cell samples from the tunicate Phallusia nigra revealed unprecedented f orms 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 I M HCl, and three previously unprecedente d chelated V(III) forms (37.5 +/-4.6%). Two of these could be represented b y the model ligand environments V(acetylacetonate)(3) (17.9 +/-3.2%) and K- 3 V(catecholate)(3) (13.1 +/-4.7%), implying DOPA-like complexation. The th ird chelated form was represented by the 7-coordinate N2O5 complex Na[V(edt a)(H2O)] (8.0 +/-1.8%). This coordination array, suggestive of a novel mono nuclear V(III) protein site, contributed only to fits to samples 1, 2, 3 an d 5, which were prepared in the presence of DTT. Endogenous V(IV) (25.2 +/- 5.4%) was principally modeled as VOCl2 in I M HCl. EPR spectra (averages: A (perpendicular to)=(1.842 +/-0.006) x10(-2) cm(-1); A(perpendicular to)=(0. 718 +/-0.007) x10(-2) cm(-1); g(parallel to)=1.936 +/-0.002; g(perpendicula r to)=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 successfull y fit the XAS pre-edge energy region. This endogenous V(V) signal is also u nprecedented. These results are compared with those of analogous fits to th e blood cells of Ascidia ceratodes and may support assignment of P. nigra t o a different genus. (C) 2001 Elsevier Science BY All rights reserved.