Vanadium in ascidians: changes in vanadium coordination and oxidation state upon cell lysis

Changes in vanadium coordination during cell lysis have been followed by EPR spectroscopy of the blood cells of the phleborbranch ascidians Ascidia ceratodes and PHallusia julinea. The spectra obtained for A. ceratodes whole blood samples can be mainly ascribed to aquated oxovanadium(IV) in which the signals are broadened in freshly frozen blood relative to when the cells are lysed by thawing. The sources of this broadening are discussed and it is shown that the oxovanadium(IV) signal has its origin in a small percentage of damaged or lysed cells which release vanadium into a low sulfate, low acid environment in fresh samples. When thawed, the cells lyse releasing acid and sulfate into the environment of the oxovanadium(IV), with consequent narrowing of the EPR spectral linewidth. Freshly frozen P. julinea blood cell samples have EPR spectra with parameters intermediate between aquated oxovanadium(IV) and the "type I" parameters observed in a previous investigation of tissue samples of this species (S. G. Brand, C. J. Hawkins, A. T. Marshall, G. W. Nette, and D. L. Parry, Comp. Biochem. Physiol. 93B, 425 (1989)). A. ceratodes tissue samples also have EPR spectra that differ from that of the blood. It is suggested that EPR studies on tissue samples are more indicative of the resting state of vanadium in the cells as there is more physiological material to provide a pH buffering effect to stabilize the cells. Schemes are presented which incorporate all of the EPR observations in ascidian literature, where cellular lysis is proposed to be accompanied by vanadium undergoing oxidation and a series of chelate exchanges from a "type I" complex to aquated oxovanadium(IV). Protons released during these exchanges are suggested to provide the acidity characteristic of blood cell lysates. The biological implications of the concomitant release of vanadium and tunichrome (S. W. Taylor, D. L. Parry, C. J. Hawkins, and J. H. Swinehart, Comp. Biochem. Physiol. 106A, 531 (1993)) from the blood cells, to the process of wound repair are discussed.