Isolation of D-myo-inositol 1:2-cyclic phosphate 2-inositolphosphohydrolase from human placenta

We have isolated D-myo-inositol 1:2-cyclic phosphate 2-inositolphosphohydrolase (EC 3.1.4.36) from human placenta. This enzyme catalyzes the conversion of inositol 1:2-cyclic phosphate to inositol 1-phosphate. The enzyme was purified 1300-fold to apparent homogeneity from the soluble fraction of human placenta. The enzyme requires Mn²⁺ or Mg²⁺ ions for activity, has an apparent K(m) for inositol 1:2-cyclic phosphate of 0.15 mM and forms 2.2 μmol of inositol 1-phosphate/min/mg protein. The enzyme does not utilize the cyclic esters of inositol polyphosphates as substrates. The molecular weight determined by gel filtration chromatography is approximately 55,000. Upon electrophoresis in polyacrylamide gels in sodium dodecyl sulfate, the molecular weight was found to be 29,000 both in the presence and absence of β-mercaptoethanol. The enzyme was inhibited by inositol 2-phosphate (IC₅₀ = 4 μM) and to a lesser degree by inositol 1-phosphate (IC₅₀ = 2 mM) and inositol (IC₅₀ = 4 mM). Zn²⁺ is a potent inhibitor of enzyme activity (IC₅₀ = 10 μM). Neither Li⁺ nor Ca²⁺ had any effect on enzyme activity. This enzyme may serve to generate inositol from inositol cyclic phosphate metabolites produced by the phosphoinositide signaling pathway in cells.