Inositol polyphosphate 4-phosphatase is inactivated by calpain-mediated proteolysis in stimulated human platelets.
Inositol polyphosphate 4-phosphatase (4-phosphatase), an enzyme that catalyzes the hydrolysis of the 4-position phosphate of phosphatidylinositol 3,4-bisphosphate, was shown to be a substrate for the calcium-dependent protease calpain in vitro and in stimulated human platelets. Stimulation of platelets with the calcium ionophore, A23187, resulted in complete proteolysis of 4-phosphatase and a 75% reduction in enzyme activity. Thrombin stimulation of platelets resulted in partial proteolysis of 4-phosphatase and a 41% reduction in enzyme activity (n = 8, range of 36-51%). In addition, preincubation with the calpain inhibitor, calpeptin, suppressed the accumulation of phosphatidylinositol 3, 4-bisphosphate in thrombin-stimulated platelets by 36% (n = 2, range = 35-37%). These data suggest that the calpain-mediated inhibition of 4-phosphatase is involved in the phosphatidylinositol 3, 4-bisphosphate accumulation in thrombin-stimulated platelets.
Norris
F A
FA
Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Atkins
R C
RC
Majerus
P W
PW
D002154:Calpain / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004151:Dipeptides / Q000494:pharmacology; D006801:Humans; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D015539:Platelet Activation / Q000502:physiology*; D013379:Substrate Specificity; D013917:Thrombin / Q000494:pharmacology
D015853:Cysteine Proteinase Inhibitors; D004151:Dipeptides; D018129:Phosphatidylinositol Phosphates; C060980:phosphatidylinositol 3,4-diphosphate; C071482:calpeptin; D010744:Phosphoric Monoester Hydrolases; C053153:phosphatidylinositol-3,4-bisphosphate 4-phosphatase; D013917:Thrombin; D002154:Calpain