The inositol trisphosphate phosphomonoesterase of the human erythrocyte membrane
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Colleges, School and Institutes
- University of Birmingham
Human erythrocyte ghosts exhibit an inositol triphosphate phosphomonoesterase activity that rapidly converts inositol 1,4,5-trisphosphate into inositol 1,4-bisphosphate and P(i). 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 K(m) for inositol trisphosphate is approx. 25 μM. 2,3-Bisphosphoglycerate is a competitive inhibitor, with a K(i) 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-triphosphate 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.
|Number of pages||9|
|Publication status||Published - 1 Jan 1982|