Paradoxical digestion of myo-inositol phosphates by alkaline phosphatase at low pH

The ability of bovine intestinal alkaline phosphatase (0.1-10 units/ml) to cleave myo-inositol bound phosphate moieties was examined. Paradoxically the digestion was optimal for a number of isomers at pH 5-7. It is possible that digestion at higher pH (9-10) does not proceed at maximal rates due to a conformation of the myo-inositol phosphate molecule which stabilizes the molecule against enzymatic attack. Alkaline phosphatase activity did not require the addition of added divalent cations. Moreover, several divalent cations, particularly zinc, were found to have a marked inhibitory effect. Further studies into this phenomenon suggested that some divalent cations can form insoluble complexes with myo-inositol phosphates, particularly those possessing a number of phosphate moieties, preventing the action of degradative enzymes. On the basis of these experiments we conclude that phosphate moieties can be removed from myo-inositol using relatively low concentrations of alkaline phosphatase as long as optimal incubation conditions are selected. This includes the use of a slightly acidic incubation media without the addition of divalent cations, particularly zinc.