Interaction of phytases with minerals and availability of substrate affect the hydrolysis of inositol phosphates

The stability and activity of phytases in the soil environment may be affected by their sorption on soil particle surfaces and by substrate availability with important consequences for P cycling and nutrient bioavailability. This work evaluated the interaction of phytases with goethite, haematite, kaolinite, montmorillonite and two oxisol clays and investigated how this interaction is affected when myo-inositol hexakisphosphate (InsP6) was sorbed on the mineral surfaces. phyA histidine acid phosphatases of fungal origin were used and their ability to release orthophosphate from the InsP6-saturated minerals was evaluated. The phytases showed a high affinity for the mineral surfaces, with a loss of enzyme activity generally being observed over 24 h (up to 95% of the initially added activity). The loss of phytase activity was dependent on the type of mineral, with kaolinite and montmorillonite showing the greatest effect. Retention of enzyme activity was higher with the two oxisol clays, suggesting that the heterogeneous nature of clay surfaces and the presence of endogenous organic matter may limit the inhibition caused by interaction with minerals. In the presence of mineral surfaces saturated with InsP6, the partitioning of enzyme activity between the solution and the solid phase was shifted more towards the solution phase, presumably due to the mineral surfaces being occupied by the substrate. However, phytases were not able to release any orthophosphate directly from InsP6-saturated goethite and haematite, and hydrolysed InsP6 that was desorbed from haematite. Conversely, in the case of kaolinite and of the oxisol clays, where desorption was limited, phytases appeared to be able to hydrolyse a small fraction of the InsP6 adsorbed on the surfaces. These findings suggest that the bioavailability of P from inositol phosphates is governed to a large extent by the mineral composition of soil and by competitive effects for sorption on reactive surfaces among inositol phosphates and phytases.