Hydrolysis of Phosphatidylinositol 4-phosphate and Phosphatidylinositol(4,5)-Bisphosphate by Specific Phosphomonoesterases in Human Erythrocytes

Summary

Evidence has been obtained for the sequential dephosphorylation of Ptdlns(4,5)P₂ in human erythrocytes by two specific phosphatases. A Mg$M{g^{{2^ + }}}$-dependent phosphatase, partially-purified from cytosol, specifically removes the 5-phosphate from Ptdlns(4,5)P₂ and lysoPtdlns(4,5)P₂. A cation-independent phosphatase which only hydrolyzes Ptdlns4P and lysoPtdlns4P is localized in the membrane. Maximal hydrolysis of exogenous substrates by both phosphatases requires cationic and nonionic detergents. Under these conditions, $C{a^{{2^ + }}}$ inhibits the Ptdlns(4,5)P₂ phosphatase but not the Ptdlns4P phosphatase by competing with $M{g^{{2^ + }}}$. Treating the membranes with ATP (or, ADP) and a divalent cation ($B{a^{{2^ + }}}$, $M{g^{{2^ + }}}$, $M{g^{{2^ + }}}$, $M{g^{{2^ + }}}$) irreversibly inactivates the Ptdlns4P phosphatase. Endogenous Ptdlns4P and Ptdlns(4,5)P₂ are degraded by these enzymes only after membranes are disrupted with Triton X-100. A similar differential location of the two activities was observed in other mammalian erythrocytes. Homogenates of various rat tissues also exhibit cation-independent Ptdlns4P phosphatase activity which, except for brain, exceeds the $M{g^{{2^ + }}}$-dependent Ptdlns(4,5)P₂ phosphatase activity.