The inositol trisphosphate phosphomonoesterase of the human erythrocyte membrane

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Abstract
Human erythrocyte ghosts exhibit an inositol trisphosphate phosphomonoesterase activity that rapidly converts inositol 1,4,5-trisphosphate into inositol 1,4-bisphosphate and Pi. Degradation of the released inositol 1,4-bisphosphate is not observed. This activity is dependent on Mg2+ (or Mn2+) and it is not activated by Ca2+. Optimum activity is .apprx. pH 7 and activity is abolished by heat denaturation. The Km for inositol trisphosphate is .apprx. 25 mM. 2,3-Bisphosphoglycerate is a competitive inhibitor, with a Ki [inhibition constant] of .apprx. 0.35 mM. Glycerophosphoinositol 4,5-bisphosphate is attacked at .apprx. 1/8 of the rate for inositol trisphosphate, but glycerophosphoinositol 4-phosphate is not a substrate. Incubation of 32P-labeled erythrocyte membranes with Mg2+ causes little breakdown of phosphatidylinositol 4,5-bisphosphate, the parent compound from which both glycerophosphoinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate are derived. As judged by its substrate specificity and the inhibition by 2,3-bisphosphoglycerate, this enzyme is apparently selective for the 5-phosphate in those water-soluble phosphate esters of inositol that possess the vicinal pair of 4,5-phosphates, but it may also interact less strongly with other water-soluble compounds that have pairs of vicinal phosphates.

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