Vanadium tunicates

Hemovanadins. Green components of certain blood cells of marine tunicates, consisting of a polypeptide with 24 vanadium ions (V ) in the molecule. H. are not oxygen transporters and their function is unknown but is purported to be related to the formation of cellulose. 100000 m of sea water contain merely ca. 1 g of vanadium which is enriched in the tunicates. Vanadium can be extracted technically from some types of crude oil (containing the remains of prehistoric tunicates). 

Vanadium is known to be essential, and is a constituent of some haloperoxidases as well as nitrogenases in some nitrogen-fixing organisms. It is particularly abundant in tunicates (a species of marine organisms) and in Amanita toadstools. 

Tunicates (ascidians or sea-squirts) are invertebrate marine organisms, which can accumulate vanadium at concentrations approaching 350 mM (the concentration of vanadium in seawater is 10 8 M). This vanadium is taken up as V(V) from seawater (Figure 17.16), reduced to oxidation state III or IV and stored in a soluble form in the blood cells within acidic vacuoles at concentrations a million fold higher than in their external surroundings. 

High concentration of vanadium in the blood of some tunicates has been a long-standing problem of biochemistry. That vanadium(III) ions are part of the respiratory pigment has been ruled out recently. The efficient mechanism used to concentrate vanadium from sea water is now understood but the utility of vanadium for these living organisms is still an intriguing question. 

The sea squirts or tunicates are fascinating marine creatures, their name being derived from the tunic made of cellulosic material that surrounds the body of the animal. In 1911, Henze discovered vanadium in the blood of Phallusia mammillata C.343 He later found the same with other ascidians (a class of tunicates). In vanadium-accumulating species, most vanadium is located in the vacuoles—vanadophores—of certain types of blood cells—the vanadocytes. The concentration in the vanadophore can be as high as 1M and this value must be compared with concentrations of the order of 2 x 10-8 M for vanadium in sea water.344 Kustin et al. have reviewed the work done to understand the efficient accumulation and the possible biological roles of the metal.345... 

The uses of the metal for the living creature have been the subject of much speculation but the possibility that vanadium(III) is involved in oxygen transport has been ruled out experimentally.352 The role of the high concentrations of vanadium in tunicates remains tantalizing. 

The phylum Chordata, to which we ourselves belong, includes not only the vertebrates but also more primitive marine animals that have a spinal cord. Among these primitive species, which may be related to early ancestral forms, are the tunicates or sea squirts. They have a very high concentration of vanadium in their blood. 

The first suggestion of a possible biochemical function for vanadium came from the discovery that vanadocytes, the green blood cells of tunicates (sea squirts), contain 1.0 M V(III) and 1.5-2 M H2S04. 613 It was proposed that a V-containing protein is an oxygen carrier. However, the V3+ appears not to be associated with proteins612 and it does not carry 02. 

The ascidians or tunicates (sea squirts) accumulate vanadium from seawater (about 5x 10-8 mol dm-3) to a level of about 1 mol dm-3 and store it in a dilute solution of sulfuric acid (pH<2) in blood cells called vanadocytes. The tunicates thus concentrate vanadium several million-fold. 079 NMR, ESR and EXAFS determinations on whole vanadocyte cells of Ascidia ceratodes and Ascidia nigra indicate that the vanadium is present mainly as aquated V111 probably complexed with sulfate. Some vanadyl ion (5-10%) is also present.1080 1081... 

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... 

Extended X-ray absorption fine structure (EXAFS) and K edge absorption spectroscopy have been used to study vanadium in coaI7). In contrast to the common belief that vanadyl porphyrins are the major compounds present, these studies demonstrate that in the high vanadium content samples examined, the metal is tetravalent and coordinated to oxygen ligands. Similar investigations have been carried out on the vanadocytes of aci-dians to identify the form of vanadium in tunicate blood cells22 (vide infra). 

After a treatment of general aspects of metals in the marine environment, specific target organisms, mainly bivalves (clams, mussels, oysters) and tunicates are considered. The latter class of marine organisms, commonly referred to as sea squirts, allows us to trace the pathway of a specific element, vanadium, from outer to inner environment. 

Laboratory studies of metal ion-ligand or metal ion-organism interactions are carried out under controlled conditions. In the marine evironment, however, organisms encounter metal ions under a variety of conditions. Most trace elements rarely occur in sea water as simple hydrated ions, in complexes with the major marine anions (Cl", HCO5, SO2"), or in complexes with dissolved organic compounds. Although these forms can serve as metal ion sources to organisms, as in the case of vanadium and tunicates, other sources are more abundant. 

From a chemical point of view, the setting for the encounter between tunicate and vanadium at the sea water interface is fairly simple to visualize. In the oxygenated, mildly alkaline, highly dilute conditions of the external sea water environment vanadium is present as monomeric vanadate anion. It is there predominantly as the free ion the low concentrations of organic chelators in sea water favor complex dissociation rather than formation (Cf. p. 141)8. ... 

Another approach to function ascribes anti-feedant effects to vanadium65. Aside from weather, adult tunicates, which are sessile (stationary) animals, are primarily vulnerable to two types of ecological threats. They may succumb to predatory crustaceans and fish, or they may be fouled i.e., covered over by growths of smaller organisms such as smaller tunicates, or algae. How do immobile tunicates survive these pressures Is it possible that they use a chemical defense to thwart their enemies If so, is vanadium their main line of defense ... 

Two types of evidence indicate that vanadium may function as a anti-feedant for unicates. First, there is evidence based on observation that species whose outer coverings (tunics) are rich in vanadocytes are shunned by predatory fish65,66. These tunics are often acidic, which confers further protection from predation on an individual specimen exhibiting this characteristic. Since vanadium would be expected to hydrolyze in sea water, this acidity could result from release of vanadium due to rupturing blood cells. Second, pieces of fish deliberately dosed with appreciable amounts of vanadium, and offered as food pellets to crustaceans and fish, resulted in reduced food consumption by these predators67. ... 

In the external environment of tunicates, most metal ions are cationic, few are anionic. Vanadium is one of the anions, as it is present predominantly as HzVOj at the pH of sea water. Only chromate, among the metal-containing anions, is significant in sea water and it is present at a fiftieth the concentration of vanadium8. Other anions to consider are sulfate and phosphate, present as SO4 and HPO4" in sea water. 

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