REACTIONS BETWEEN TUNICHROME MM-1, A TUNICATE BLOOD PIGMENT, AND VANADIUM IONS IN ACIDIC AND NEUTRAL MEDIA

摘要

Tunichromes are yellow, polyphenolic tripeptides prevalent in blood cells of tunicates (suborders phlebobranchia and stolidobranchia). Spectrophotometric studies of reactions between tunichrome Mm-1 and V-V or V-IV 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 V-IV in pH 7 buffer, both catechol and Mm-1 formed complexes with V-IV ions, and no redox products were found. For V-V in pH 2 buffer, both catechol and Mm-1 were oxidized by V-V ions. Room temperature EPR qualitatively showed that Mm-1 in pH 2 buffer reduced V-V ions to free V-IV ions. For V-V in pH 7 buffer, Mm-1 was oxidized by V-V ions and formed V-IV complexes. At higher concentrations, the V-IV 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 V-III, we found that reactions between Mm-1 and V-V or V-IV ions in pH 7 buffer clearly did not generate appreciable quantities of V-III products. Thus, the colorimetric V-III assay resolved the issue of V-III 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.