Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 9, 2002
Publication Date: January 5, 2003
Citation: DAO, T.H. PLOYVALENT CATION EFFECTS ON MYO-INOSITOL HEXAKISPHOSPHATE ENZYMATIC DEPHOSPHORYLATION IN DAIRY WASTEWATER. JOURNAL OF ENVIRONMENTAL QUALITY. 2003.
Interpretive Summary: Substantial amounts of organic phosphorus stored in feed grains are not available to the animal and contribute to water pollution rather than animal productivity. Organic phytate accounts for 60 to 90% of the stored reserves of P in grains that include wheat, barley, rice, corn, and soybean. Forages and grains are fed to livestock in various proportions, depending upon stages of growth and energy requirements of their life cycl and production goals. Phytase in saliva catalyzes the breakdown of feed grain and fiber phytate to inorganic P that is absorbed primarily in the animal gut. However, swine and poultry lack the phytase enzyme to breakdown phytate and excrete most of this form of organic phosphorus. Ruminant livestock, thought able to utilize phytate-P because of microbial phytase in the rumen, also excrete a large portion of phytate. Remediation strategies or technologies to reduce excessive P levels in manure and P- loaded soils depend on our understanding of the release of organic P of fiber and grains that are the major components of manure. Calcium was found to activate the dephosphorylation of phytate in dairy manure at low concentration. However, as Ca or Al or Fe content increased, the degradation of phytate and release of soluble P were severely reduced, averaging 30% less at ratio of 1 to 6 Al or Fe to phytate-P and reaching over 99% when metal to phytate-P ratio was on a 1 to 1 basis. Therefore, the nature of the counterion associated with phytate controls phytate physical state in solution and its susceptibility to enzymatic hydrolysis that may lead to phytate persistence in manure and potentially where manure is land applied.
Information is needed on the contribution of organic polyphosphate phytate to the sources and sinks of dissolved phosphorus in the soil-manure-water system that is often associated with water quality degradation in watersheds with confined animal feeding operations. Effects of Na+, Ca2+, Al3+, and Fe3+ and three cation to phytate-P mole ratios on phytate enzymatic dephosphorylation were determined in dairy manure containing fro 1 to 100 g L-1 of total solids. Phytate dephosphorylation by Aspergillus ficuum phytase EC 184.108.40.206 followed first-order decay kinetics at 20 oC. As counterion concentration increased and Ca2+ to phytate-P mole ratios reached 6:6, dephosphorylation decreased by 50 (# 3.6) and 40 (# 4) % at ph 4.5 and 6, respectively. Polyanionic phytate had high affinity for Al3+ and Fe3+ and reduction in dephosphorylation averaged 27 and 32% at a mole ratio of 1:6 and reaching over 99% at mole ratio of 6:6, for Al3+ and Fe3+, ,respectively. In dairy manure, a phytase-hydrolyzable P fraction was native and proportional to manure solid concentration. The differential protective effects of cations influenced the accuracy of estimates of the PHP fraction in manure. Low Ca2+ concentrations did not interfere with IP6 net dephosphorylation in manure while Ca2+ at 6:6 Ca to IP6-P mole ratio, Al3+ and Fe3+ severely inhibited the reaction. Increased manure TSS also decreased IP6 susceptibility to dephosphorylation. Therefore, the nature of the counterion and its mole ratio to P control phytate physical state in solution and its susceptibility to enzymatic hydrolysis that may lead to phytate persistence in manure and potentially where manure is land applied.