Phospholipase A2 activity of rat stomach

Phospholipase A activity in rat stomach wall and in gastric content was studied using [1-14C]dioleoylphosphatidylcholine as substrate. The optimum activity of the stomach wall was found to take place at pH 7.0. During optimal phospholipase action about 40% of the [1-14C]oleic acid released was due to an active intracellular lysophospholipase. The gastric phospholipase required 5 mM Ca2+ for full activity and is inhibited by EDTA. It specifically hydrolyzed the sn-2 position of the phospholipid molecule. The enzyme was heat labile and inactivated by acidification at pH 3.0. The gastric content enzyme had a lower specific activity and an optimum pH of 8.0. It was heat stable and was not inactivated by acidification. These results indicate that gastric content phospholipase A is of pancreatic origin, via a duodenal reflux. By ligating the stomach we were able to further confirm that the gastric wall phospholipase was different from that of the gastric content. It originated from the stomach mucosa. Subcellular fractionation suggests that the gastric phospholipase A2 is essentially bound to the plasma membrane. About 6% of the activity was found to be soluble. Biopsies of human gastric mucosa displayed a phospholipase A activity which had similar properties to that of rat gastric enzyme. The physiological function of this enzyme is discussed in terms of prostaglandin synthesis via the release of arachidonic acid.