Tunichromes

Recently, a reducing blood pigment named tunichrome B-l (10) was isolated from the tunicate Ascidia nigra L. and characterized.351 It has a structure derived from three (3,4,5-trihydroxy)phenylalanine units and the trihydroxyphenyl group is suitable for reducing the metal. 

A chromogen has been isolated from several species (tunichrome), but its role is not understood.1082.1083... 

A completely new source of electrochemiluminescent compounds may have been recently discovered as well. Weak luminescence was observed from fluids extracted from tunicates, a type of marine invertebrate, and synthetic tu-nichromes (the chromophore in the tunicates) upon oxidation [66], The structure of the synthetic tunicate chromophore is shown in Fig. 13. A 10-fold enhancement of ECL is observed when the synthetic analogues of the tunichromes are complexed to Hg2+. The mechanism of the luminescent enhancement has not yet been elucidated. 

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... 

This photochemically mediated cisjtrans isomerization of enamines has been used to synthetic advantage by Nakanishi and coworkers in a synthesis of the tunichromes Mml and Mm22. These workers observed that the stereochemistry of the enamine formed by the Emmons reaction shown in Scheme 1 could be changed from EjZ = 8/1 to EjZ =1/2 on irradiation. The undesired E isomer could then be separated and recycled. It should be noted, however, that cis/trans isomerization is more typically an undesired competitive relaxation pathway for most of the photochemical transformations described in this chapter. 

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.

Extraction chromatography, through which cellular contaminants such as carotenoids are removed and a concentrated crude extract of a mixture of tunichromes is prepared. 

Centrifugal counter-current chromatography, which provides a relatively rapid, nondestructive technique for separating individual tunichromes. Great losses are encountered during this phase of the procedure. 

Preparative high-pressure liquid chromatography, which is the only technique currently available that has sufficiently high resolving power to separate a pair of tunichromes. 

The resulting purified material is more stable than the impure samples and could be characterized by a variety of spectroscopic and chemical conversion techniques. In addition, an assay of crude blood-cell extract for the presence of tunichrome was developed (162). Tunichrome blood pigments consist of a number of closely related polyphe-nolic compounds with a central triglycyl unit (Fig. 5). A tunichrome (designated An-1) isolated from the species A. nigra, for example, of... 

Fig. 5. The structures of tunichromes (blood pigments) from A. nigra consist of a number of closely related compounds (a) designated TC An-i (i = 1, 2,. . . ) those obtained from M. manhattensis (b) are designated TC Mm-i.

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. 

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