Myo-inositol hexakisphosphate

S ATP -I- 1 D-myo-inositol hexakisphosphate <1> (<1>, enzyme is responsible for the biosynthesis of diphospho-myo-inositol pentakisphosphate. The enzyme also has a ATP synthase activity, implying that 5-diphos-pho-1 D-myo-inositol pentakisphosphate functions as high-energy phosphate donor [1]) (Reversibility r <1> [1]) [1]... 

Phytase offers significant promise as a means to reduce phosphorus levels in animal waste by 30-35%, while also reducing the cost of phosphorus supplementation. The enzyme hydrolyzes phytate (myo-inositol hexakisphosphate), the primary storage form of phosphorus in plant seeds and pollen, in several steps into inositol and inorganic phosphorus, which is readily bioavailable to the farm animals. Phytases can also have non-specific phosphorus monoester activity. Addition of phytases to farm animals diets significantly enhances bioavailability of plant phosphorus for the animals while reducing phosphorus in the waste and simultaneously allowing a reduction of total phosphoms in the feed 500 units of phytase... 

Phytase phosphatase Aspergillus niger var. (1) myo-inositol- hexakisphosphate-3- phosphohydrolase (2) orthophosphoric-mono ester phosphohydrolase 3.1.3.8 3.1.3.2... 

Irigoin, F., Ferreira, F., Fernandez, C., Sim, R.B., and Diaz, A., 2002, myo-inositol hexakisphosphate is a major component of an extracellular structure in the parasitic cestode Echinococcus granulosus. Biochem J. 362 297-304. 

Greiner, R., Farouk, A., Alminger, M.L., and Carlsson, N.G., 2002, The pathway of dephosphorylation of myo-inositol hexakisphosphate by phytate-degrading enzymes of different Bacillus sp. Can. J. Microbiol. 48 986-994. 

Gel filtration chromatography has been widely used to separate high and low molecular mass phosphorus in sediments, soil solutions, and preconcentrated surface water. Steward and Tate (1971) showed, for example, that Sephadex G50 could be used to separate high molecular mass phosphorus that corresponded with the molecular weight of myo-inositol hexakisphosphate. Others used Sephadex gel filtration separations to demonstrate the presence and importance of inositol phosphates in preconcentrated lake water (Eisenreich and Armstrong, 1977), algal cultures (Minear, 1972),... 

Fig. 1.4. Gradient elution ion-exchange chromatography of an inositol phosphate standard mixture. Concentrations (mg/l) phosphate (PO ) = 25, inositol monophosphate (IMP) = 28, myo-inositol hexakisphosphate (IHP) = 34. Reproduced from Minear etal. (1988) with permission from The Royal Society for Chemistry.

Turner, B.L., Mahieu, N. and Condron, L.M. (2003c) Quantification of myo-inositol hexakisphosphate in alkaline soil extracts by solution P NMR spectroscopy and spectral deconvolution. Soil Science 1 68, 469 78. 

The inositol hexa- and pentakisphos-phates are prevalent in soils compared to lower-order esters, probably because stability in the soil is linked to the number of phosphate groups. The most widespread stereoisomer is myo-inositol (Dalai, 1977). Inositol phosphates are more resistant to mineralization than the other fractions of the soil organic phosphorus and, therefore, are probably poorly available to plants (Williams and Anderson, 1968). They are present in soils in highly complex forms associated with clay minerals, fulvic and humic acids (Anderson and Arlidge, 1962), proteins and some metallic ions (Rojo et al., 1990). The various forms of inositol phosphates are often imprecisely referred to as phytic acid, which is reserved exclusively for the free acid form of myo-inositol hexakisphosphate. Salt forms of myo-inositol hexakisphosphate, also known as phytates, are very stable and consequently accumu-... 

Activation energy values for the hydrolysis of myo-inositol hexakisphosphate and of esters with lower phosphate content were between 35.6 kJ/mol in germinated Phaseolus aureus (Mandal et al., 1972) and 50.2 kJ/mol in wheat bran (Nagai and Funahashi, 1962). The optimum temperature for the enzymatic hydrolysis of myo-inositol hexakisphosphate varies among phytases between 45 and 57°C (Nayini and Markakis, 1986), and even up to 60°C for the phytase of Bacillus subtilis (Powar and Jagannathan, 1982). 

The lowest and the highest values for myo-inositol hexakisphosphate hydrolysis were reported in phytases of Aspergillus ficuum (pH 5.3, 0.01 mM) and in that of germinated Phaseolus aureus (0.65 mM), respectively. The highest values reported were of wheat bran phytase towards myoinositol tetrakis- and trisphosphate (5 mM see Nayini and Markakis, 1986). K and K, values for the enzymatic hydrolysis of myoinositol hexakisphosphate by different Bacillus spp. were determined to be approximately 0.44 mM and 18.6/s, respectively. The affinity of myo-inositol pentakisphosphate for the phytase enzymes and their maximal rates of hydrolysis were lower (K = 0.50-0.76 mM 7.4-16/s) than that of myo-inositol hexakisphosphate (Greiner et al., 2002). 

The classical method of measuring phosphatase activity by the hydrolysis of para-nitrophenyl phosphate could be irrelevant for estimating the potential hydrolysis of natural organic phosphorus compounds such as myo-inositol hexakisphosphate, since the relative activity of phytase compared to para-nitrophenyl phosphatase activity may be several orders of magnitude lower (Beck et al., 1989 Joner et al., 2000 Tib-bett, 2002). 

Barrow (1993) successfully applied this model to phosphate adsorption by taking into account pH and electrolyte concentration. The use of this equation to describe organic phosphorus adsorption can be important as well, due to the high charge density of molecules such as myo-inositol hexakisphosphate, but the complexity of organic phosphorus compounds and the different mechanism of adsorption can make the model difficult to apply. 

Fig. 6.3. Isotherms of adsorption of myo-inositol hexakisphosphate (IHP) on goethite, ferrihydrite, illite, kaolinite, calcite and ferrihydrite-kaolinite mixed systems at pH 4.5 and in 0.01 M KCI (adapted from Celi etaL, 1999,2000,2003).

Fig. 6.4. Mechanism of adsorption of myo-inositol hexakisphosphate on the goethite surface at pH 4.5. For simplicity, the hydrogen and hydroxide groups of the phosphate molecule are omitted (Ognalaga et ai, 1994).

In soils and sediments, complexation can increase organic phosphorus stabilization, especially with iron (III) and calcium ions and their minerals (Harrison, 1987 House and Denison, 2002). The interaction with iron (III) was reported to transform a large part of the labile and moderately labile organic phosphorus forms supplied with manure to paddy soils into more resistant organic phosphorus, possibly because inositol phosphates initially bound to calcium or magnesium were transformed into iron-bound compounds (Zhang et aL, 1994). In the presence of calcium, myo-inositol hexakisphosphate can form two soluble calcium complexes with one or two calcium ions (Ca - or Ca2-phytate), but when three calcium ions are involved (Cag-phytate), the complex precipitates at all pH values (Graf, 1983). This enhances the interaction of myo-... 

Fig. 6.6. Titration curves of myo-inositol hexakisphosphate (Phy) alone and in the presence, at 6 1 molar ratio, of zinc (II), copper (II), calcium (II), cobalt (II) and nickel (II) (Martin and Evans, 1987).

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