Phospholipid Metabolism of Rat Gastric Mucosa

Phospholipids of rat gastric mucosa comprised about 30% of the total extractable lipidsl phosphatidylcholine (PC) constitutes about half of the phospholipids. Disaturated (dipalmitoyl)-PC made up about 31% of the PC species, a concentration comparable to that reported for lung tissue. Gastric mucosa exhibited three distinguishable phospholipid deacylating enzyme activites; lysophospholipase, phospholipase A 1 and phospholipase A 2 and an acylase activity owing to a lysophospholipase-transacylase enzyme complex. The lysophospholipase hydrolyzed 1-palmitoyl lyso-PC to free fatty acid and glycerophosphorylcholine (GPC). This enzyme had an optimum pH of 8.0, was heat labile, did not require Ca 2+ for maximum activity and was not inhibited by bile salts or buffers of high ionic strength. Phospholipase A 2 and phospholipase A 1 deacylated dipalmitoy1-PC to the corresponding lyso compound and free fatty acid. The specific activity of phospholipase A 2 was 2-4-fold higher than that of phospholipase A 1 under all the conditions tested. Both activities were enhanced 4-7.5-fold in the presence of bile salts at alkaline pH and 11-18-fold at acidic pH. Phospholipase A 2 hydrolyzed diarachidonyl-PC>PC-18:2>>PC-18:1>PC-16:0. No phospholipase C or D activities were detected with dipalmitoyl1-PC as substrate. The lysophospholipase-transacylase enzyme comples displayed both a lysophospholipase and a transacylase activity. The transacylase converted, stoichiometrically, 2 moles of 1-palmitoyl1 lyso-PC to one mole each of dipalmitoyl-PC and GPC in the absence of ATP, CoA, fatty acids, bile salts or detergents. The transacylase-lyso-phospholipase activities were both located in the cytosol fraction and were associated at a constant ratio through-out several purification steps, including the isoelectrofocusing procedure. The high levels of dipalmitoyl-PC and the presence of deacylating enzymes and of a lysophospholipase-transacylase enzyme complex in gastric mucosa and the lung suggest similarities between gastric and lung phospholipid metabolism. Dipalmitoyl-PC is generally accepted as the major active component of pulmonary surfactant, but the exact role of dipalmitoyl-PC in gastric mucosa awaits furthere documentation. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.