A Possible Pathway of Phosphoinositide Metabolism Through EDTA-Insensitive Phospholipase A1Followed by Lysophosphoinositide-Specific Phospholipase C in Rat Brain

Incubation of [2-3H]glycerol-labeled phosphatidylinositol with a crude cytosol fraction of rat brain in the presence of EDTA yielded [3H]lysophosphatidylinositol predominantly without accumulation of labeled monoacylglycerol and diacylglycerol. The pH optimum of this phospholipase A activity was 8.0. The activity for phosphatidylinositol was twofold higher than for phosphatidylethanolamine, whereas phosphatidylcholine, phosphatidylserine, and phosphatidic acid were not hydrolyzed significantly under the conditions used. The phospholipase A activity for phosphatidylethanolamine was resolved in part from that for phosphatidylinositol by ammonium sulfate fractionation of the cytosol, indicating the existence of at least two forms of EDTA-insensitive phospholipase A. The positional specificity of the phosphatidylinositol-hydrolyzing activity was found to be that of a phospholipase A1, as radioactive lysophosphatidylinositol was produced from 1-stearoyl-2-[1-14C]arachidonyl-sn-glycero-3-phosphoinositol without release of free arachidonate. A phospholipase C activity specific for lysophosphoinositides was found in a membrane fraction from rat brain, which was similar to that characterized in porcine platelets. The phospholipase C was demonstrated to hydrolyze the 2-acyl isomer as well as the 1-acyl isomer of lysophosphatidylinositol. Taken together, our results suggest a possible pathway through which phosphatidylinositol is selectively degraded to the 2-acyl isomer of lysophosphatidylinositol in a Ca(2+)-independent manner, and subsequently converted to a 2-monoacylglycerol in rat brain.