Catalyzes the hydrolysis of inorganic orthophosphate from phytate. Releases 5 of the 6 phosphate groups to yield the end product myo-inositol 2-monophosphate. Specific for phytate. 1D-myo-inositol hexakisphosphate + H2O = 1D-myo-inositol 1,2,4,5,6-pentakisphosphate + phosphate Optimum pH is 5.5. Active from 2.5 to 7.5 with phytic acid as substrate. The optimum pH is shifted to more acidic values with 4-nitrophenyl phosphate as substrate. Monomer. Belongs to the histidine acid phosphatase family.