Purification and characterization of a 43-kDa membrane-associated inositol polyphosphate 5-phosphatase from human placenta

We have identified, isolated, and characterized a membrane-associated inositol polyphosphate 5-phosphatase (5-phosphatase) from the particulate fraction of human placenta. The enzyme was purified 3700-fold from a detergent extract of human placental membranes to apparent homogeneity, by chromatography on DEAE-Sepharose, S-Sepharose, hydroxylapatite, and Biosil SEC 250 HPLC gel filtration. The purified 5-phosphatase has a molecular mass of 43 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration chromatography. The enzyme hydrolyzes inositol 1,4,5-trisphosphate (Ins(1,4,5)P₃) to inositol 1,4 bisphosphate (Ins(1,4)P₂) with an apparent K(m) of 5 μM. The 43-kDa 5-phosphatase also hydrolyzes inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P₄) with an apparent K(m) of 1.2 μM. The enzyme requires Mg²⁺ ions for activity and is inhibited by Ca²⁺ concentrations greater than 100 μM. Polyclonal antibodies developed against the membrane-associated enzyme immunoprecipitate the purified membrane-associated placental 5-phosphatase and the platelet Type I cytosolic enzyme, but not the 75-kDa platelet Type II 5-phosphatase. These results demonstrate that the purified membrane 5-phosphatase bears physical and immunological similarity with the Type I cytosolic platelet enzyme.