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Phosphatase activity, using p-nitrophenyl phosphate(NPP) as the substrate, was inhibited by myo-inositol hexaphosphate(phytate). The extent of inhibition by 5 HIM phytate of the alkaline phosphatase activity of calf intestine and Escherichia coli, and of the acid phosphatase activity of wheat germ was 82, 56 and 50%, respectively. However, acid phosphatases from rice bran were not inhibited by 10mM phytate. The effect of preincubation with phytate on the acid and alkaline phosphatase activity was investigated. After a 180-min preincubation with 1 mM phytate, the extent of inhibition was 98% for the calf intestine enzyme, 88% for the E. coll enzyme, and 75% for the wheat germ enzyme. Mg^<2+> enhanced the phosphatase activity, but the addition of phytate lowered it. The inhibition of E. coli alkaline phosphatase by phytate was noncompetitive, with an apparent inhibitor constant of 0.45 HIM under the assay conditions. The inhibition of calf intestine alkaline phosphatase and of wheat germ acid phosphatase was of the mixed type, with inhibitor constants of 0.13niM and 0.43 μM, respectively. Therefore, phytate may interact with a complex of these phosphatases and substrates.