Cereal- and Microbial Phytases. Phytate Degradation, Mineral Binding and Absorption

Cereal foods contain large amounts of phytate (myo-inositol hexaphosphate; IP₆), which is the major storage form of phosphorus, in the plant. IP₆ possesses a high potential for chelating minerals, such as Fe ²⁺, Zn²⁺, Mg²⁺ and Ca²⁺. This is of nutritional importance owing to the negative effect on the bioavailability of essential elements from the food. During food processing, IP₆ can be completely or partly degraded. Thus, processed foods may contain a mixture of different inositol phosphates, with 6 or fewer phosphate groups, IP₆-IP₁.<p /> This thesis deals with IP6, its degradation during breadmaking, the mineral binding capacity of its degradation products and the effect of cereal- and microbial phytases on IP₆ hydrolysis and dietary iron absorption in humans.<p /> The binding of metal ions (Cu²⁺, Zn²⁺ and Cd²⁺) to isolated fractions of inositol phosphates (IP₆-IP₃) was investigated at pH 3-7. All inositol phosphate fractions studied had a pronounced binding capacity at pH 5 - 7. This indicates that mineral complex formation with lower inositol phosphates (IP₄, IP₃) may occur in the duodenum, which is important from a nutritional point of view if these complexes inhibit mineral absorption. Phytases are phosphatases, capable of hydrolyzing IPV, thus producing inositol phosphates with a lower degree of phosphorylation. The effects of cereal (wheat) and microbial (Aspergillus niger and Saccaromyces cerevisiae) phytases on IP₆ hydrolysis were studied in wheat during breadmaking. Activation of endogenous wheat phytase, the addition of Baker's yeast (S. cerevisiae) and, in particular, phytase from A. niger reduced IP₆ during breadmaking.<p /> A specific method for determining phytase activity by measuring the formation IP₅ was developed. As different phytases produce different isomers of IP₅, this method can also be used to distinguish between phytases of different origins.<p /> Commercial strains of S. cerevisiae proved able to develop phytase activity, i.e. to grow on a synthetic medium with IP₆ as the only source of phosphorus. Both strains studied expressed a metabolism well adapted for utilization of phosphorus from IP₆. The phytases proved to be of the 3-phytase type, yielding DL-Ins(1,2,4,5,6)P₅, DL-Ins(1,2,5,6)P₄ and DL-Ins(1,2,6)P₃ as degradation products.<p /> The potential of wheat phytase and A. niger phytase to improve iron absorption from a meal with phytate was studied in humans using a radioisotope technique. When added to the meal at consumption, endogenous wheat phytase had no effect but A. niger phytase was found to markedly increase iron absorption