Degradation of phosphatidylinositol by soluble enzymes of rat gastric mucosa.

Rat gastric mucosa homogenates contain two enzymatic systems for hydrolyzing phosphatidylinositol: a deacylation activity yielding lysophosphatidylinositol and free fatty acid, and a phospholipase C-like activity producing 1,2-diacylglycerol and inositol phosphates. These activities were found mainly in the 105 000 x g supernatant and could be distinguished by differential stabilities, metal requirements and the action of deoxycholate and mepacrine. Each lipolytic reaction displayed a major pH optimum at 7.5 and a minor pH optimum at 5.5. The deacylation system was 8-10 times as active as the phospholipase C, with an apparent Km of 0.63 mM towards 1-acyl-2-arachidonylphosphatidylinositol at pH 7.5. The phospholipase C activity, on the other hand, hydrolyzed 1-acyl-2-arachidonylphosphatidylinositol or 1-acyl-2-arachidonylphosphatidylethanolamine and yielded 1-acyl-2-arachidonyl-sn-glycerol. This 1,2-diacylglycerol could be phosphorylated to form 1-acyl-2-[14C]arachidonyl-sn-phosphoglycerol (phosphatidic acid), but could not be hydrolyzed to produce free [14C]arachidonic acid using stomach mucosal microsomes. Phospholipase A2 and phospholipase C attack 1-acyl-2-arachidonylphosphatidylethanolamine and phosphatidylinositol equally well, but hydrolyze 1-acyl-2-arachidonylphosphatidylcholine poorly.