The isolation and characterization of inositol polyphosphate 4-phosphatase

We previously identified an alternative pathway for the metabolism of inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) in calf brain. The enzyme responsible for the degradation of Ins(1,3,4)P3 was designated as inositol polyphosphate 4-phosphatase (Bansal, V. S., Inhorn, R. C., and Majerus, P. W. (1987) J. Biol. Chem. 262, 9644-9647). We have now purified this enzyme 3390-fold from calf brain-soluble fraction. The isolated enzyme has an apparent molecular mass of 110 kDa as determined by gel filtration. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the enzyme migrates as a protein of 105 kDa, suggesting that it is monomeric. Among various 4-phosphate-containing inositol polyphosphates, the enzyme hydrolyzes only Ins(1,3,4)P3 and inositol 3,4-bisphosphate (Ins(3,4)P2), yielding inositol 1,3-bisphosphate and inositol 3-phosphate as products. The inositol polyphosphate 4-phosphatase has apparent Km values of 40 and 25 microM for Ins(1,3,4)P3 and Ins(3,4)P2, respectively. The maximum velocities for these two substrates are 15-20 mumol of product/min/mg protein. Ins(1,3,4)P3 is a competitive inhibitor of Ins(3,4)P2 hydrolysis with an apparent Ki of 27 microM implying that the same active site is involved in hydrolysis of both substrates. The final enzyme preparation retained a small inositol polyphosphate 3-phosphatase activity (less than 2% of rate of inositol polyphosphate 4-phosphatase activity) which most likely reflects a contaminant. The enzyme displays maximum activity between pH 6.5 and 7.5. It is not inhibited by Li+, Ca2+, or Mg2+ except at 10 mM divalent ions. Mn2+ inhibits enzyme at high concentrations IC50 = 1.5 mM.