Vanadium in tunicate blood cells

C. Biological Origin Vanadium in Tunicate Blood Cells... 

By using radioisotopes, the influx of vanadate into the blood cells of A. nigra has been studied85. The influx of phosphate, sulfate and dichromate, and the inhibitory effect of these oxoanions on vanadate influx have also been determined. The rate of vanadate influx was measured in the presence of metabolic inhibitors and inhibitors of anion transport. In this study, EPR spectroscopy was used to follow changes in the concentration of reduced vanadium within tunicate blood cells exposed to vanadate. 

The EPR technique has been used extensively to identify and characterize oxo-vanadium(IV) (i.e., vanadyl) in tunicate blood cells. Although EPR spectra are clearly diagnostic of vanadyl, different species yield different values of vanadyl EPR parameters (A and g values). Different values of EPR parameters have been reported for species such as A. ceratodes 144, 145), Leptoclinides lissus and Phallusia julinea 143). Vanadyl EPR Aq values of the blood cells of A. ahodori collected from different locations in the waters off Japan have been reported to differ from one another by approximately 10% (1.06 x 10 cm vs. 0.95 X 10 cm ), an amount about as much as interspecies differences 146). This result has been questioned, however, in the context of an extensive study of both Aplousobranch and Phlebobranch species utilizing several different methods to detect intracellular vanadium 147). 

Elements other than vanadium are accessible to the EXAS technique. Thus, abundant sulfur detected in tunicate blood cells has been identified as sulfate and sulfonate with EXAS (150). This result confirmed a similar finding carried out with chromatographic techniques (151). With this background, let us now consider the X-ray microprobe analyses of timicate blood cells. 

The occurrence of vanadium in the lower oxidation states, which as the simple aqua ions undergo acid dissociation above pH 3 [if present as V(III)) ] and pH 6 [in the case of oxo-V(IV) ], along with the high sulfur content of ascidian blood and the low pH that results when ascidian blood cells are ruptured in distilled water has led to the belief that intact vanadium-containing tunicate blood cells are acidic (145). Other lines of evidence, including vital staining and NMR (144,170), and... 

Other examples of vanadium occurrence in nature are the relatively high concentrations of vanadium in the blood cells of tunicates, and in the fly agaric kman ta mu taKjLa). In both cases the vanadium enrichment is especially interesting, and propably the most spectacular example of metal enrichment in nature. 

The blood of ascidians is distributed tbrougbout tbe body, tunic included, by tbe peristaltic movement of tbe heart and through a complex vascular system. The presence of high levels of vanadium in the blood cells of the Mediterranean species Phallusia mamillata, coupled with the extreme acidity of the blood (pH < 2), was reported by... 

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. 

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 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. ... 

Vanadium is an essential trace element to humans it is also essential to tunicates. However, these marine organisms concentrate vanadium within their blood cells, where the element is present in molar concentrations and the term trace element is no longer appropriate. 

Frank P, Hedman B, Hodgson (1999) Sulfur allocation and vanadium-sulfate interactions in whole blood cells from the tunicate Ascidia ceratodes, investigated using X-ray absorption spectroscopy. Inorg Chem 38 260-270... 

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... 

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. ... 

Francis JT, Enkvist C, Lunell S, Hitchcock AP (1994) Studies of X Is to n triplet states of carbon monoxide, benzene, ethylene, and acetylene. Can J Phys 72 879-884 Frank P, Hedman B, Carlson RMK, Hodgson (1994) Interaction of vanadium and sulfate in blood cells from the tunicate Ascidia ceratodes Observations using X-ray absorption edge structure and EPR spectroscopies. Inorg Chem 33 3794-3803... 

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