Hydrolysis of precipitated phytate by three distinct families of phytases

While genetically modified plants that secrete histidine acid phosphatases (HAPs), β-propeller phytases (BPPs) and purple acid phosphatases (PAPs) have been shown to assimilate soluble phytate, little is known about whether these plants have the ability to hydrolyze precipitated phytate. In this study, the ability of representative members of these three classes of phytases to hydrolyze metal-phytate salts and to hydrolyze phytate adsorbed to aluminum precipitates was compared. All three phytases were able to hydrolyze Ca²⁺-, Mg²⁺-, and Mn²⁺-phytates, but were unable to hydrolyze Al³⁺-, Fe²⁺-, Fe³⁺-, Cu²⁺-, and Zn²⁺-phytates. When these ions were present, the hydrolysis of Ca²⁺-phytate was prevented. Citrate was more potent than malate and oxalate in solubilizing some of these phytate salts for enzyme hydrolysis. Phytate adsorbed to aluminum precipitates was resistant to all three enzymes, except when organic acids were added (citrate>oxalate>malate). While increasing concentrations of organic acids were inhibitory to enzyme activity (oxalate >citrate>malate), PAP was more resistant to citrate than HAP. As desorption of phytate from a solid surface by organic acids is essential for phytase activity, the genetic engineering of plants that enhances the secretion of both citrate and phytases from the root may be a feasible approach to improving soil phytate assimilation.