Vanadate-phosphate system

In the vanadate-phosphate system, the mixed species form,... 

From the experimental results, which will be described later in this chapter, it appears that the condensed compounds of phosphate-silicate, vanadate-phosphate, and arsenate-phosphate are hydrolyzed, when dissolved in water, by the scission of P-O-X linkages and, in some cases, X-O-X linkages. Monomer and polymer units of PO4 tetrahedra inserted between two atoms of X are released into the solution when the system is dissolved in water. Consequently, orthophosphate and/or chain phosphates are produced. As for condensed compounds of sulfate-phosphate, it has been found that copolymers of sulfate-phosphate persist in aqueous solutions. 

Ohashi and Matsumura (51) have investigated the NaVOs-NaPOs system and presumed the existence of copolymers of vanadate and phosphate, which have V-O-P linkages. Both glasses and crystals of alkali meta-arsenate-phosphates, when dissolved in water, decompose by the scission of As-O-P linkages to give arsenate and mixtures of condensed phosphates (see Section V). As is the case with condensed arsenate-phosphates, it is assumed that condensed vanadate-phosphates, if present, decompose by the scission of V-O-P linkages, when dissolved in water, to give vanadate and mixtures of condensed phosphates. 

As already mentioned, a part of vanadium(V) of sodium vanadate-phosphates is reduced to vanadium(IV), when the components are melted. A similar phenomenon has been observed by Munakata and his co-workers (31). They have found that a considerable amount of low valency vanadium, probably vanadium(IV), is produced when glasses of the Ba0-V205-P206 system are prepared. The amount of vanadium(IV) decreases with the increase of the Ba0/P205 ratio and of the total amount of vanadium present in the glass and increases with... 

Indeed, the observation that vanadate is a potent inhibiter of phosphate-recognizing enzyme systems was a great stimulus to work in this area, but it now seems likely that its action is more complicated than simple mimicry of phosphates.This is germane to obtaining an understanding of the antitumor activity of [V( j -C5H5)2Cl2]. 

Gresser MJ, Tracey AS (1990) Vanadates as Phosphate Analogs in Biochemistry. In Chastcen ND (ed) Vanadium in Biological Systems. Physiology and Biochemistry. Kluwer Academic, Dordrecht, p 63... 

Vanadate is taken up by N. crassa when the cells are phosphate limited and growing in higher pH values. It takes place by the phosphate transport system II, which has high affinity for phosphate. Phosphate is a competitive inhibitor of vanadate uptake.1071 This confirms the competitive relationship between phosphate and vanadate. 

Vanadate is reduced in red cells to vanadyl ion by intracellular glutathione following uptake through a phosphate transport system.1072,1073 Free vanadyl is normally unstable with respect to oxidation, but appears to be stable when complexed with intracellular proteins or smaller molecules.1074 Vanadyl ion is a relatively powerful inhibitor of (Na+, K+)-ATPase. For pure fractions of the enzyme, inhibition was nearly complete at less than 5 pmol dm-3 vanadyl ion.1075 The state of vanadyl ion at pH 7 is somewhat uncertain, but may involve a hydroxylated species. Vanadyl ion also inhibits alkaline phosphatase more effectively than does vanadate.1076... 

Selective oxidation materials fall into two broad categories supported systems and bulk systems. The latter are of more practical relevance although one intermediary system, namely vanadia on titania [92,199-201], is of substantial technical relevance. This system is intermediary as titania may not be considered an inert support but rather as a co-catalysts [202] capable of, for example, delivering lattice oxygen to the surface. The bulk systems [100, 121, 135, 203] all consist of structurally complex oxides such as vanadyl phosphates, molybdates with main group components (BiMo), molybdo-vanadates, molybdo-ferrates and heteropolyacids based on Mo and W (sometimes with a broad variation of chemical composition). The reviews mentioned in Table 1.1 deal with many of these material classes. 

Despite the fact that the formation of these alkoxo and aryloxo vanadates is not favored relative to many other ligated species, they can still have important influences in enzymic systems. It has, for instance, been shown that vanadate in the presence of glucose and glucose-6-phosphate dehydrogenase readily produces gluconic acid, a normal product of glucose-6-phosphate metabolism [3], Similar reactivity has been observed with a number of enzymes that metabolize phosphate compounds [4],... 

Gresser, M.J., A.S. Tracey, and N.D. Chasteen. 1990. Vanadates as phosphate analogs in biochemistry. Vanadium in biological systems. Dordrecht, Boston, London Kluwer Academic Publishers, pp 63-79. 

Bowman, B.J. 1983. Vanadate uptake in Neurospora crassa occurs via phosphate transport system II. J. Bacteriol. 153 286-291. 

Simulation of glucose transport and glucose transporter translocation from intracellular stores to the plasma membrane in muscle cells by vanadate and peroxovan-adate involve a mechanism independent of PI-3K and protein kinase C systems utilized for stimulation of these processes by insulin. The transport of GLUT4 to the plasma membrane in muscle cells growing in culture after stimulation by vanadate, peroxovanadate, or insulin all require an intact actin network [138], Sometimes, the insulin-like action of vanadium is accompanied by overall stimulation of actual metabolic pathways. One example of this is the stimulation of the pentose phosphate pathway observed when vanadate promotes the incorporation of glucose into lipids, an antilipogenic effect [139],... 

Plass, W. (1999). Phosphate and vanadate in biological systems chemical relatives or more Angewandte Chemie, 38, 909-912. 

Two-dimensional and three-dimensional structures containing analogous and related macrocyclic ring systems are known for most of the ligand types that form discrete macrocyles. Additionally, there are a few examples for two-dimensional and three-dimensional coordination polymers containing sulfate, phosphate, vanadate, chromate, molybdate, and tungstate, indicating that these are also possible candidates for the formation of discrete macrocycles. 

Table 2.5 provides an overview of the concentrations of important components of human blood, important in the sense that they constitute potential ligands for vanadium. The concentrations of lactate (1.5 mM) and citrate (0.1 mM) are comparatively high. Along with phosphate (1.2 mM), they are the major potential low molecular weight llgators for vanadate and vanadyl. The system containing vanadate(V) and lactate (lac) has been extensively smdled,[ °] and the species Identified on the basis of NMR and H+ potentiometry Include mono-, di-, trl- and tetranuclear forms, for which the structures... 

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