Phospholipid-deacylating enzymes of rat small intestinal mucosa.

1. Two phospholipase activities, provisionally designated as phospholipase activity I and phospholipase activity II, were found to be present in the mucosal homogenates of rat small intestine. These phospholipase activities were present in the membraneous particle fraction and were characterized in this study without further purification, using phosphatidylcholine as a substrate. Phospholipase activity I was assayed at pH 5.9 in the absence of deoxycholate, whereas phospholipase activity II was assayed at pH 9.4 in the presence of deoxycholate. Phospholipase activity I was more easily inactivated by heat treatment and trypsin digestion than phospholipase activity II. Both phospholipase activities were inhibited by diisopropyl-fluorophosphate but not by SH-binding reagents. 2. Phospholipase activity I had a pH optimum at 5.9. A sigmoid curve was obtained when the amount of the enzyme preparation was plotted against the phospholipase activity I. The unusually low activity found at low enzyme concentrations was enhanced by addition of the heat-inactivated enzyme preparation to a level where a linear relationship was found between the amount of enzyme and the activity. The effector present in the enzyme preparation was tentatively identified as fatty acid(s). The addition of oleic acid or linoleic acid to the incubation mixture enhanced the phospholipase activity I. At 1 mM levels of these fatty acids the highest activity was obtained when 1.5 mM phosphatidylcholine was used as a substrate. 3. The phospholipase activity II increased on addition of deoxycholate. In the presence of 5 mM deoxycholate, a pH optimum was found at 9.6. It was found that the maximal extent of hydrolysis of phosphatidylcholine in the incubation mixture was dependent on the concentration of deoxycholate. This indicates that deoxycholate facilitates the action of phospholipase activity II, presumably by forming deoxycholate-phosphatidylcholine mixed micelles. Phospholipase activity II was found to deacylate specifically the 2-acyl moiety of phospholipids.