Subcellular localization of the enzymes that dephosphorylate myo-inositol polyphosphates in human platelets

The phosphatase-induced hydrolysis of [3H]inositol 1,4-bisphosphate [Ins(1,4)P2)] and [3H]inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was studied in platelet subcellular fractions. The activity that hydrolyses Ins(1,4)P2 is cytosolic, whereas the activity that hydrolyses Ins(1,4,5)P3 is present in both particulate and cytosolic fractions. The cytosolic Ins(1,4)P2 phosphatase hydrolyses the 1-phosphate of Ins(1,4)P2, whereas the cytosolic and membrane-bound Ins(1,4,5)P3 phosphatases hydrolyse the 5-phosphate of Ins(1,4,5)P3. In the presence of ATP, it is possible to observe a cytosolic Ins(1,4,5)P3 3-kinase that phosphorylates Ins(1,4,5)P3 to inositol 1,3,4,5-tetrakisphosphate. Apparent Km values for the particulate and the cytosolic Ins(1,4,5)P3 phosphatases are 100 microM and 40 microM respectively. A large proportion of the membrane-associated Ins(1,4,5)P3 phosphatase can be extracted with 1 M-NaCl, and the Mr of this enzyme, as determined by hydrodynamic studies, is 49,000, whereas that of the cytosolic enzyme is 59,000. The Km values for the cytosolic Ins(1,4)P2 phosphatase is 40 microM; this enzyme has an Mr of 49,000. The highest specific activity of the Ins(1,4,5)P3 phosphatase is present in a highly purified plasma-membrane fraction.