Vanadium tunichromes

Two possible models for putative vanadium-tunichrome interaction in ascidians [V "acac (bipy)catCl4] [acac = acetylacetonate(l —) bipy = bipyridyl catCl4 = tetrachlorocatecholate(2—)]P 1 and [V O(gly-sal)cat] [gly-sal(l—) is the Schiff base formed from glycine and sabcylaldehyde cat = catecholate(l—... 

Martoja, R., Gouzerh, P., and Monniot, F. (1994) Cytochemical studies of vanadium, tunichromes and related substances in asddians possible biological significance. Oceanog. Mar. Biol. Anna. Rev., 32, 531-556. 

The tunichromes constitute a class of amino-acid-derived metabolites isolated from the blood cells of ascidians. The chemicals are involved in vanadium sequestration and reduction in the blood cells and may be involved in a primitive clotting mechanism to repair damaged tissue.226-227 The... 

Examples of secondary metabolites from Ascidians are shown in Figure 3.10. Ascidians are noteworthy for the presence of vanadium (and related metals) together with high concentrations of sulphuric acid and tunichromes, which are unstable hydroquinoid compounds. A defensive role for these has been suspected,98 and it is noteworthy that gastropod molluscs (both prosobranch and opisthobranch) that feed upon tunicates often secrete large amounts of sulfuric acid, used in both... 

A chromogen, tunichrome, has been isolated from the cells of several species and has been partially characterized114,115). In A. nigra, tunichrome is present in near equimolar amounts with vanadium and is capable of reducing vanadate(V). The precise role of tunichrome is not known. It may participate in the accumulation and reduction of vanadium by the cell. A model for cellular uptake of vanadium has been proposed113,114. ... 

Fig. 4. Frequently cited ascidian blood cell types. The middle column refers to staining properties of each type of cell with the two reagents osmium tetroxide and the pH indicator neutral red. Free tunichrome would reduce osmium tetroxide, as would lower oxidation states of vanadium.

Underivatized ( )-tunichrome An-1 was synthesized recently on a semi-preparative scale 163). Its availability will help to clarify the biological role of tunichromes, including their interactions with vanadium. 

The X-ray microanalytical studies suggest that accumulated vanadium is not found in aqueous intracellular volumes such as vacuoles but occurs mainly in hydrophobic granules and membranes. Chelation by a mixture of ligands including water, sulfate, and tunichrome could have two effects on the lower oxidation states of vanadium, namely, to decrease the reduction potential and increase the stabilization at physiological pH. The EPR line-width studies (145), carried out on whole blood samples and based on aqua vanadyl models, may simply be inappropriate or applicable to only a fraction of the acciunulated vanadium. 

The amount of vanadium found in tunicates also appears to correlate with the pH (more vanadium at lower pH). Individuals belonging to the Phiebobranchia usually also contain tunichromes, whereas Apiousobranchia are devoid of this hydroxy-DOPA-based oligopeptide pigment (see below). The vanadium is present in special blood cells, called vanadocytes, essentially represented by two cell types the signet ring cells and the vacuolated amoebocytes.P 1 They accumulate vanadium from sea water [c(V) >= 30 nm] by a factor of up to 10. ... 

It appears to be well established by now that the reductant responsible for the conversion of vanadate(V) to oxovanadium(IV) is nicotine adenine dinucleotide phosphate (NADPH), produced in the pentose phosphate pathway.PI Further reduction to in the vacuoles of the vanadocytes of Phlebobranchia ascidians remains elusive. Several reductants have been proposed and shown to be effective in the in vitro and/or in vivo reduction of to V and/or to including tunichromes. Selected investigations of the related redox chemistry of vanadium can be briefly summarised as follows ... 

M. manhattensis does not, however, accumulate vanadium. No appreciable reduction to V was found with the tunichrome from A nigra, an effective vanadium accumulator (Table 4.1). On the other hand, reaction of [VO(acac)2] with 2,4-di(fert-butyl)catechol yields [V(semiquinone)3]. 

Catechols (and pyrogallols) readily reduce vanadium(V) to vanadium(IV) and, in some instances, further to vanadium(III). In the context of tunichromes as the presumed reducing agents in ascidians, the redox chemistry of catecholatovanadium complexes has been investigated to some extent. Results on reduction potentials for the and... 

Vanadium is often taken up as vanadate, in a pathway parallel to phosphate. However, its oxidation state within organisms seems to be highly variable. Unusually high concentrations of vanadium occur in certain ascidians (the specific transport behavior of which will be dealt with later). The workers who first characterized the vanadium-containing compound of the tunicate, Ascidia nigra, coined the name tunichrome. The characterization of the compound as a dicatecholate has been reported. 

Here it is reduced to or [VO] + by the polyphenolic blood pigment tunichrome. (Note the structural relationship between tunichrome and l-DOPA, 26.5.) Storage of vanadium must involve the formation of or [VO] + complexes, but the nature of these species is not known. 

The tunichromes are a group of reducing blood pigments from tunicates which were initially implicated in metal ion accumulation (particularly vanadium and iron), and have been recently reviewed [34]. They are dipeptides constructed from 2P(3,4,5-trihydoxyphenyl)alanine (Topa) or 2p(3,4-dihydoxyphenyl)aIanine (Dopa) and a C-terminally bound 3,4-dihydroxystyrylamine or 3,4,5-trihydroxy-styrylamine unit, and while it is now postulated that small proteins which are rich in Dopa and/or Topa units accumulate the metal ions rather than the tunichromes themselves,... 

Oltz. E.M. Bruening, R.C. Smith, M.J. Kustin. K. Nakanishi, K. The tunichromes. A class of reducing blood pigments from sea squirts Isolation, structures and vanadium chemistry. J. Am. Chem. Soc. 1998,110. 6162-6172. 

Vanadium in Ascidians and the Chemistry of Tunichromes Mitchell J. Smith, Daniel E. Ryan, Koji Nakanishi,... 

Isol. from the tunicate Ascidia nigra. Blood pigment which selectively accumulates vanadium. rZ-Isomer [97689-87-7]. Tunichrome BI C26H25N3O11 M 555.497 Isol. from A. nigra. Dec. in air on warming. 

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