Identification of multiple phosphoinositide-specific phospholipases D as new regulatory enzymes for phosphatidylinositol 3,4, 5-trisphosphate

In the course of delineating the regulatory mechanism underlying phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) metabolism, we have discovered three distinct phosphoinositide-specific phospholipase D (PI-PLD) isozymes from rat brain, tentatively designated as PI-PLDa, PI-PLDb, and PI-PLDc. These enzymes convert 3H]PI(3,4,5)P3 to generate a novel inositol phosphate, D-myo-3H]inositol 3,4,5-trisphosphate (3H]Ins(3,4,5)P3) and phosphatidic acid. These isozymes are predominantly associated with the cytosol, a notable difference from phosphatidylcholine PLDs. They are partially purified by a three-step procedure consisting of DEAE, heparin, and Sephacryl S-200 chromatography. PI-PLDa and PI-PLDb display a high degree of substrate specificity for PI(3,4, 5)P3, with a relative potency of PI(3,4,5)P3 > phosphatidylinositol 3-phosphate (PI(3)P) or phosphatidylinositol 4-phosphate (PI(4)P) > phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) > phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2). In contrast, PI-PLDc preferentially utilizes PI(3)P as substrate, followed by, in sequence, PI(3,4,5)P3, PI(4)P, PI(3,4)P2, and PI(4,5)P2. Both PI(3, 4)P2 and PI(4,5)P2 are poor substrates for all three isozymes, indicating that the regulatory mechanisms underlying these phosphoinositides are different from that of PI(3,4,5)P3. None of these enzymes reacts with phosphatidylcholine, phosphatidylserine, or phosphatidylethanolamine. All three PI-PLDs are Ca2+-dependent. Among them, PI-PLDb and PI-PLDc show maximum activities within a sub-microM range (0.3 and 0.9 microM Ca2+, respectively), whereas PI-PLDa exhibits an optimal Ca2+] at 20 microM. In contrast to PC-PLD, Mg2+ has no significant effect on the enzyme activity. All three enzymes require sodium deoxycholate for optimal activities; other detergents examined including Triton X-100 and Nonidet P-40 are, however, inhibitory. In addition, PI(4,5)P2 stimulates these isozymes in a dose-dependent manner. Enhancement in the enzyme activity is noted only when the molar ratio of PI(4,5)P2 to PI(3,4, 5)P3 is between 1:1 and 2:1.