Vanadium accumulation mechanism

Figure 14 Mechanism of vanadium accumulation in the vanadophore assuming a membrane permeable only to anions...

Michibata, H., T. Uyama, and K. Kanamori. 1998. The accumulation mechanism of vanadium by ascidians. In Vanadium compounds. Chemistry, biochemistry and therapeutic applications, A.S. Tracey and D.C. Crans (Eds.), American Chemical Society, Washington, D.C., pp. 248-258. 

Only the features of vanadium biochemistry relevant to understanding the accumulation mechanism in vanadocytes are presented here. More extensive coverage of this subject may be found in two recent reviews51 525. 

Seefeldt, L. C., Hoffman, B. M., Dean, D. R. (2009). Mechanism of Mo-dependent nitrogenase. The Annual Review ofBiochemistry, 78,701—722. Ueki, T, Furano, N., Michibata, H. (2011). A novel vanadium transporter of the Nramp family expressed at the vacuole of vanadium-accumulating cells of the ascidian Ascidia sydneiensis samea Biochimica et Biophysica Acta, 1810, 457—464. 

The catalyst is preliminarily oxidized to the state of the highest valence (vanadium to V5+ molybdenum to Mo6+). Only the complex of hydroperoxide with the metal in its highest valence state is catalytically active. Alcohol formed upon epoxidation is complexed with the catalyst. As a result, competitive inhibition appears, and the effective reaction rate constant, i.e., v/[olefin][ROOH], decreases in the course of the process due to the accumulation of alcohol. Water, which acts by the same mechanism, is still more efficient inhibitor. Several hypothetical variants were proposed for the detailed mechanism of epoxidation. 

For the vanadophore region, the sulfate concentration has been estimated to be of the order of 1.3 M and pH 2.349 With no evidence for a stable complex within the vanadocyte, Figure 14 depicts an interesting mechanism proposed for vanadium and sulfate accumulation.350 Anionic vanadium(V) (as HVO ") and sulfate ions enter the cell. Provided that within the vanadophore there is a strong reducing agent, vanadium can be reduced to vanadium(IV) and vanadium(III), cationic at the low pH in the vanadophore. If the vanadophore membrane is permeable to anions but not cations, the reduced vanadium remains trapped. 

Those submicron particulates which enter the alveolar sacs may undergo various degrees of absorption, depending upon the solubility of their components, or are transported to the base of the ciliated bronchiolar epithelium (54). Alveolar absorptive efficiency for most trace elements is 50-80% (50). Retention or absorption is not necessarily a simple function of solubility. Silver iodide, for example, is rapidly absorbed from the lungs even though it is weakly soluble in water (56). Likewise, insoluble elemental lead deposited in the respiratory passages is absorbed, but the mechanism involved remains to be elucidated (49). Vanadium probably accumulates in human lungs in insoluble forms... 

At the present time, considerable insight into the biochemistry of vanadium in tuni-cates is associated with the mechanism of vanadium uptake. We shall concentrate on this topic, first presenting the biological context in which to view this remarkable feat of metal ion accumulation. 

Catalytic hydrogenations of aromatic nitro compounds with a stable hydroxylamine intermediate often have two different kinetic phases hydrogen uptake is rapid up to ca 60 %, then distinctly slower in the second phase. This means that reduction of the hydroxylamine to the aniline, formally a hydrogenolysis, is difficult in these cases. In the presence of the promoters discussed in Section 8.5.4.3, the second phase is less pronounced or disappears. This suggests a mechanism which could be called catalytic by-pass (see Figure 4). Experiments in the absence of hydrogen indicated that the vanadium promoters catalyze the disproportionation to give aniline and the nitroso intermediates that re-enter the catalytic cycle. As a consequence, the hydroxylamine does not accumulate and aniline formation is accelerated. 

H. Michibata The Mechanism of Accumulation of Vanadium by Ascddians - Some Progress Towards an... 

The measurement results of electrical resistance of irradiated films (Figure 24.19) speak in favor of the offered mechanism. At irradiation doses of up to 10 cm" both materials manifest an increase in resistance provoked by the accumulation of helium at boundaries, which results in enlargement of electron scattering centers. Further, chromium nitride experiences a catastrophic growth of resistance, which results in collapse. The vanadium nitride forms a stable system of opened channels, through which the excess helium escapes. The value of electrical resistance remains unchanged. 

Michibata, H. (1996) The mechanism of accumulation of vanadium by ascidians some progress towards an understanding of this unusual phenomenon. Zool. Sci, 13, 489-502. 

It is still not apparent whether vanadium is an essential element in humans. However, there are marine organisms, ascidians and worms, which accumulate vanadium (Crans et al, 2004). These organisms accumulate the vanadium(V) present in seawater and convert it into either vanadium(IV) or vanadium(III) for storage within the organisms, and consequently, hydrolytic reactions involved in the mechanism are important. The function of vanadium in these organisms remains elusive even though many credible hypotheses have been postulated (Crans etal., 2004). 

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