Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 23, 2003
Publication Date: February 1, 2004
Citation: DAO, T.H. 2004. Organic ligant effects on the enzymatic dephosphorylation of myo-inositol hexakis dihydrogenphosphate in dairy wastewater. Journal of Environmental Quality. 33(1):349-358.
Interpretive Summary: Organic phosphorus, primarily as phytate accounts for 60 to 90% of the stored reserves of P in grains and forages that are fed to livestock depending upon stages of growth and energy requirements. Phytic acid is largely indigestible to swine and poultry. A large fraction was also found in manure of ruminant livestock, although microorganisms of the rumen possess necessary enzymes to breakdown and assimilate organic phytate-P. The organic phosphate is a potential significant source of dissolved P to cause quality problems in US waters. Resistance to enzymatic breakdown was shown to be caused by the formation of insoluble polymeric forms of calcium phytate, aluminum phytate, or iron phytates when phytate reacts with these chemical species as they occur in dietary mineral supplements or in the environment of manure storage pits or soils where manure is applied. Accurate information is needed on the amount of organic and inorganic phosphorous forms in manure to prescribe effective manure treatment to reduce manure soluble phosphorus that cause water quality degradation in lakes and streams in watersheds with high concentration of animal production, and economically important estuaries such as the Chesapeake Bay or the Puget Sound. The resistance to enzymatic dephosphorylation may be overcome by organic chelate ligands that render phytate forms more soluble in animal wastewaters. Organic matter, such as polyphenols, humate and other compounds of microbial degradation of plant materials may change the biological environment of phytase enzymes. These changes more likely will vary widely across animal manure handling and storage environments. Thus chelateligands can induce significant changes in bioavailability of phytate in the manure-water, and therefore manure water-soluble P pool size and environmental dispersion of soluble P.
Information is needed on the effects of other organic ligands on the enzymatic dephosphorylation of phytate (IP6) as the organic polyphosphate is a potential significant source of dissolved PO4-P in the soil-manure-water system. The effects of CDTA, EDTA, DTPA, oxalate (OXA), and phthalate (PHTH) and LIGND-to-IP6 charge concentration ratios on the enzymatic dephosphorylation of IP6 were studied to determine controlling mechanisms of IP6 persistence in manure. Solution PO4-P concentrations were analyzed by high-performance liquid chromatography. Uncomplexed IP6 dephosphorylation by Aspergillus ficuum phytase EC 126.96.36.199 at pH 4.5 and 6 and 20 oC is unaffected by the five LIGNDs. As polyvalent counterion concentrations increase, the inhibition of IP6 dephosphorylation lessens in the presence of organic LIGNDs, in the following order: CDTA = EDTA > DTPA >> OXA > PHTH. Whether CDTA or EDTA is the most effective at dissociating polyvalent counterion and IP6, allowing dephosphorylation to proceed, depends upon the suspension's acidity and LIGND-to-IP6 charge concentration. These LIGNDs and DTPA reduce the inhibitory effect of polyvalent counterions, to the point of promoting hydrolysis of the manure phytase-hydrolyzable P fraction. Therefore, the experimental results suggested that, although the fate and environmental effects of IP6-P were regulated by pH-controlled enzyme activity and IP6 water solubility by the associated counterions, IP6 resistance to enzymatic dephosphorylation may be overcome by other organic ligands in dairy wastewaters to contribute to the pool of bioavailable environmental P.