Reactions between Vanadium Ions and Biogenic Reductants of Tunicates:  Spectroscopic Probing for Complexation and Redox Products in Vitro

Abstract

Several species of marine tunicates store oxygen-sensitive VIII in blood cells. A sensitive colorimetric VIII assay was used to survey the leading candidates for the native reducing agent of vanadate in tunicates (i.e., An-type tunichromes, glutathione, NADPH, and H2S) in reactions with VV or VIV ions under anaerobic, aqueous conditions at acidic or neutral pH. Except for the case of An-1 and VV ions in pH 7 buffer, the assay results for the biogenic reducing agents clearly showed that appreciable quantities of VIII products were not generated under the conditions tested. Therefore, the assay results place new limits on hypothetical mechanisms of VIII formation in vivo. For reactions between An-1 and VV ions in pH 7 buffer, low levels of VIII products could not be ruled out because of an interfering peak in the colorimetric assays. For similar reactions between VV ions and An-1, or an An-1,2 mixture, in mildly to moderately basic media, the product mixtures precipitated as greenish black solids. Analyses of the precipitated V/An mixtures using vanadium K-edge X-ray absorption spectroscopy (XAS) showed that the major products were tris(catecholate)-type VIV complexes (65 +/- 6%) and bis(catecholate)-type VIVO complexes (20 +/- 4%). XAS analysis of the V/An-1 product mixture also provided evidence of a minor VIII component (9 +/- 5% of total V), notable for possible relevance to tunicate biochemistry. The combined results of XAS studies, spectrophotometric studies [Ryan, D. E., et al. (1996) Biochemistry 35, 8640-8650], and EPR studies [Grant, K. B., et al. (1996) J. Inorg. Biochem. (manuscript in preparation)] consistently establish that reactions between tunichromes (Mm-1 or An-1) and VV ions generate predominantly VIV-tunichrome complexes in neutral to moderately basic aqueous media.

The Other Topa: Formation of 3,4,5-Trihydroxyphenylalanine in Peptides

“…There have been reports, for example, of the tyrosinase-catalyzed formation of 3,4,5-Topa from free Dopa (4). The formation of Topa from tyrosine-containing peptides may provide insights into two different Topadependent pathways: (1) an oxidative catabolic route for degrading damaged phenolics, e.g., possibly 3,4,5-Topa from cnidarian tissues, and (2) a synthetic route for producing functional Topa side chains, e.g., Topacontaining peptides from tunicate hemocytes for vana- dium binding and reduction (19) or protein cross-linking (15,23).…”

Vanadium haloperoxidases: From the discovery 30 years ago to X-ray crystallographic and V K-edge absorption spectroscopic studies

“…The XANES of these organisms, when compared to that of suitable model compounds, showed that the vanadium was predominantly in the V(3+) state in an octahedral environment, while the EXAFS revealed the nearest neighbours to be oxygens. In subsequent studies, the V XANES features were used to quantify the amounts of V(3+) ions associated with the biological catecholate-based chelator tunichrome, and water and sulphate, as well as mixtures of the latter in various species [114][115][116][117][118], and a detailed mechanism for the reduction of vanadate to V(3+) was proposed [119]. In this particular context, the role of sulphur in the vanadium metabolism was also investigated with sulphur K edge XANES [120][121][122].…”

Absence of Mn-Centered Oxidation in the S2 → S3 Transition: Implications for the Mechanism of Photosynthetic Water Oxidation

“…17,18,43,45,46,[74][75][76][77][78][79][80] Figure 3 shows the 2nd derivatives of the pure S-state spectra from Figure 2A compared to the 2nd derivatives for each flash sample. The spectra of the individual flash samples, shown in Figure 3A, demonstrate the reproducibility achieved in this experiment.…”