Evidence for a specific phosphatidylinositol 4-phosphate phosphatase in human erythrocyte membranes

Human erythrocyte membranes exhibit a specific phosphatidylinositol 4-phosphate phosphohydrolase (PtdIns4P phosphatase) activity which hydrolyzes PtdIns4P and lysoPtdIns4P but not phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) or lysoPtdIns(4,5)P2. Phosphatidic acid, lysophosphatidic acid, glycerophosphoinositol 4-phosphate, glycerophosphoinositol 4,5-bisphosphate, inositol mono, bis, and tris phosphates and several other sugar and nucleoside phosphates are not hydrolyzed. The PtdIns4P phosphatase activity is not affected by Ca2+ or Mg2+ ions nor inhibited by EDTA. Maximum in vitro activity requires non-ionic (Triton X-100) detergents. The phosphatase is very stable in isolated membranes at low temperatures but is rapidly inactivated above 35 degrees C. This critical inactivation temperature is lowered to 20-25 degrees C by solubilizing the membranes with non-ionic detergents. Arrhenius plots of the activity show an inflection at these critical temperatures, suggesting a temperature-dependent change in the environment or conformation of the enzyme. Sulfhydryl-reacting reagents are potent inhibitors. Dithioerythritol stimulates only when the membranes are solubilized with non-ionic detergent. The location of cation-independent PtdIns4P phosphatase activity in the membrane and of Mg2+-dependent PtdIns(4,5)P2 phosphatase activity in the cytosol was also observed for monkey, rabbit, rat, and dog erythrocytes. Both activities are located in the cytosol of sheep erythrocytes.