Impact of Assay Conditions on Activity Estimate and Kinetics Comparison of Aspergillus niger PhyA and Escherichia coli AppA2 Phytases

Aspergillus niger PhyA and Escherichia coli AppA2 are increasingly used in animal feed for phosphorus nutrition and environmental protection. The objective of this study was to determine the impacts of assay conditions on activity estimates of these two phytases and to compare their biochemical characteristics at a pH similar to the stomach environment. The activities of the unpurified AppA2 were more variable than those of PhyA with three commonly used phytase activity assays. The variations associated with AppA2 were accounted for by buffer, pH, and the inclusion of Triton X-100 and BSA by approximately one-third each. At the commonly observed stomach pH of 3.5, the purified AppA2 had a lower affinity to phytate (a higher Km), but greater V-max, k(cat), and k(cat)/K-m than those of PhyA. In summary, differences between AppA2 and PhyA in responses to activity assay conditions and in inherent kinetic properties should be considered in interpreting their feeding efficacy.