PURIFICATION AND BIOCHEMICAL-CHARACTERIZATION OF A MAMMALIAN PHOSPHATIDYLINOSITOL 3,4,5-TRISPHOSPHATE 5-PHOSPHATASE

Characterization of the enzymes involved in metabolism of 3-phosphoryl ated inositol lipids and their subcellular localization revealed that in vitro a 5-phosphatase activity was responsible for the degradation of phosphatidylinositol 3,4,5-trisphosphate, whereas a 3-phosphatase a ctivity hydrolyzed phosphatidylinositol 3-phosphate and/or phosphatidy linositol 3,4-bisphosphate. All these activities were localized in the cytosol. No phospholipase activities were detected. The cytosolic pho sphatidylinositol 3,4,5-trisphosphate 5-phosphatase activity was purif ied to near homogeneity using ion exchange, affinity, and size exclusi on chromatography. Characterization of the purified phosphatase reveal ed that it is a magnesium-dependent 5-phosphatase that is able to hydr olyze phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3 ,4,5-trisphosphate. The enzyme is only partially inhibited by neomycin and vanadate but is strongly inhibited by phosphatidylinositol 4,5-bi sphosphate and to a slightly lesser extent by phosphatidylinositol 4-p hosphate.