Biochemical changes in phosphorus compounds and in the activity of phytase and α-amylase in the rice (Oryza sativa) grain during germination

Changes in the phytic acid, inorganic phosphorus and ATP contents, and in the activity of phytase and α-amylase in rice (Oryza sativa L) grains were determined during 18 days of germination in a dark room. The effect of phytic acid on α-amylase activity was studied in vitro. Rice grains immersed in sterilised deionised water at 14^°C germinated on the fifth day. Phytase activity, detected in the ripening rice grains, increased linearly until the eighth day and reached a maximum on the tenth day. There was a marked decrease in phytate and an increase in inorganic phosphorus accompanying germination. There was a good inverse correlation between the levels of both phytase activity and inorganic phosphorus, and phytate breakdown. α-Amylase activity was detected on the fourth day and increased markedly from the 12th to the 16th day of germination. ATP level increased from the second to the fourth day and slightly decreased from the fourth to the eighth day; it increased rapidly again from the eighth to the 18th day of germination. α-Amylase activity was influenced by both pH and phytic acid concentration in the assay system. At 75 mM phytic acid, α-amylase activity was lowered by 23%, 93% and 52% at pH 4–0, 5–0 and 6–0 respectively. When the enzyme, phytate and Ca²⁺ were incubated together at pH 5–0, the inhibition of α-amylase by phytic acid was markedly decreased by addition of Ca²⁺. The chemical affinity of Ca²⁺ for phytic acid was higher in the reaction at pH 5–0 than in those at pH 4–0 and pH 6–0, and over 98% of Ca²⁺ in the reaction system was precipitated as Ca-phytate.