Organic ligand effects on enzymatic dephosphorylation of myo-inositol hexakis dihydrogenphosphate in dairy wastewater

Animal manure contains partially digested feed fiber and grains where phosphorus (P) is bound in organic compounds that include myo-inositol 1,2,3,5/4,6-hexakis dihydrogenphosphate or phytic acid (IP6). Information is needed on the effects of other (non-IP6) organic ligands (LIGND) on the enzymatic dephosphorylation of IP6, which is a potential source of dissolved orthophosphate P (PO4-P) in the soil-manure-water system. The effects of 1,2-cyclohexane diamino-tetraacetate (CDTA), diethylenetriamine-N,N,N',N',N'-pentaacetate (DTPA), ethylenediamine-N,N,N',N'-tetraacetate (EDTA), oxalate (OXA), and phthalate (PHTH) and LIGND to IP6 molar ratio and charge concentration ratio on IP6 dephosphorylation were studied to determine controlling mechanisms of IP6 persistence in manure. Solution PO4-P concentrations were analyzed by ion chromatography as the phosphomolybdate-ascorbic acid method partly includes IP6-P. Uncomplexed IP6 dephosphorylation by Aspergillus ficuum (Reichardt) Henn. phytase EC 3.1.3.8 at pH 4.5 and 6 is unaffected by the presence of LIGNDs. As the concentrations of Ca2+, Al3+, or Fe3+ increase, dephosphorylation is reduced. Their inhibitory effect lessens in the presence of LIGNDs, in the following order: CDTA = EDTA > DTPA > OXA > or = PHTH. Whether CDTA or EDTA is the most effective LIGND depends upon the acidity of the suspension and LIGND charge concentration, reducing the inhibitory effect of polyvalent counterions to the point of promoting the hydrolysis of a manure phytase-hydrolyzable phosphorus (PHP) fraction that is otherwise unavailable. Therefore, ligand-induced changes increase the mobilization and dephosphorylation of complexed organic P, above and beyond the simple dissolution of inorganic phosphates. An analytical method for potentially bioavailable PHP in animal manure should include a LIGND as extracting reagent. Also, potential LIGNDs in an organic carbon-rich dairy wastewater may increase the release of PHP and environmental dispersion of PO4-P.