Reactions between tunichrome Mm-1, a tunicate blood pigment, and vanadium ions in acidic and neutral media

Tunichromes are yellow, polyphenolic tripeptides prevalent in blood cells of tunicates (suborders phlebobranchia and stolidobranchia). Spectrophotometric studies of reactions between tunichrome Mm-1 and VV or VIV ions were conducted in vitro in various media to crudely approximate cellular conditions: deionized water, aqueous methanol, and aqueous buffers at pH 2 and 7. Catechol was used in parallel studies for comparison to tunichrome and was found to be a good model for tunichrome reactivity. For VIV in pH 7 buffer, both catechol and Mm-1 formed complexes with VIV ions, and no redox products were found. For VV in pH 2 buffer, both catechol and Mm-1 were oxidized by VV ions. Room temperature EPR qualitatively showed that Mm-1 in pH 2 buffer reduced VV ions to free VIV ions. For VV in pH 7 buffer, Mm-1 was oxidized by VV ions and formed VIV complexes. At higher concentrations, the VIV complexes were observed by low temperature EPR [Grant, K. B. (1994) Dissertation, Columbia University; Grant, K. B., et al. (1996) J. Inorg. Biochem. (manuscript in preparation)]. Using a colorimetric assay for VIII, we found that reactions between Mm-1 and VV or VIV ions in pH 7 buffer clearly did not generate appreciable quantities of VIII products. Thus, the colorimetric VIII assay resolved the issue of VIII product formation raised in EPR studies of Mm-1 [cf. Ryan, D. E., et al. (1992) Biochemistry 35, 8651-8661]. Overall, the results provide insights into tunichrome-vanadium chemistry and identify conditions which promote complexation and/or redox reactions in vitro.