Hydrolysis of phosphatidylinositol by pancreas and pancreatic secretion

1. The secretion from sheep pancreas and a supernatant fraction prepared from the gland contained an EDTA-insensitive acid phospholipase A1 which readily deacylated phosphatidylinositol (pH optimum, 5.3), 1-acylglycerophosphoinositol and phosphatidic acid, but had limited action of phosphatidylcholine and phosphatidylethanolamine even with deoxycholate present. The enzyme was not a triacylglycerol lipase. 2. The action of the phospholipase A1 on phosphatidylinositol was inhibited effectively by Ca2+ and Mg2+, probably by interaction of these ions with the substrate. 3. In the presence of calcium the decomposition of phosphatidylinositol and lysophosphatidylinositol by the supernatant fraction was overwhelmingly by phosphodiesterase action (EC 3.1.4.10), producing inositol monophosphate and its cyclic derivative. Its pH optimum was about 6.0 but with considerable activity extending to pH 8.5. 4. The phosphodiesterase was not secreted in the pancreatic juice.