title: The inositol trisphosphate phosphomonoesterase of the human erythrocyte membrane.
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 around pH 7 and activity is abolished by heat denaturation. The Km for inositol trisphosphate is approx. 25 microM. 2,3-bisphosphoglycerate is a competitive inhibitor, with a Ki of approx. 0.35 mM. Glycerophosphoinositol 4,5-bisphosphate is attacked at about one-eighth of the rate for inositol trisphosphate, but glycerophosphoinositol 4-phosphate is not a substrate. Incubation of 32P-labelled 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. On the basis of its substrate specificity and the inhibition by 2,3-bisphosphoglycerate, we suggest that this enzyme is selective for the 5-phosphate in those water-soluble phosphate esters of inositol that possess the vicinal pair of 4,5-phosphates but that it may also interact less strongly with other water-soluble compounds that have pairs of vicinal phosphates.
mesh_terms: D002413:Cations, Divalent / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D004910:Erythrocyte Membrane / Q000201:enzymology*; D004912:Erythrocytes / Q000201:enzymology*; D006801:Humans; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000378:metabolism; D000072181:Inositol Polyphosphate 5-Phosphatases; D007700:Kinetics; D010744:Phosphoric Monoester Hydrolases / Q000037:antagonists & inhibitors / Q000097:blood*; D013347:Subcellular Fractions / Q000201:enzymology; D013379:Substrate Specificity
chemical_list: D002413:Cations, Divalent; D007295:Inositol Phosphates; D015544:Inositol 1,4,5-Trisphosphate; D010744:Phosphoric Monoester Hydrolases; D000072181:Inositol Polyphosphate 5-Phosphatases