Phytases are meso-inositol hexaphosphate phosphohydro-lases that catalyze the initiation of the stepwise phosphate splitting of phytic acid or phytate to lower inositol phosphate esters and inorganic phosphate (Fig. 1). These enzymes have emerged as effective tools to improve phosphorus nutrition and to protect the environment from phosphorus pollution in animal production. Although phosphorus is an essential nutrient to all species, 60 80% of phosphorus in feeds of plant origin is in the form of phytate (myo-inositol hexakisphosphate) that is poorly available to simple-stomached animals such as swine, poultry, and the preruminant calves, due to the lack of phytases in their gastrointestinal tracts. As a result, a large portion of feed phosphorus is not utilized by them and ends up in manure, causing environmental pollution. Meanwhile, expensive and nonrenewable inorganic phosphorus needs to be added to diets for these species to meet their nutrient requirements for phosphorus.

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