Phospholipid-deacylating enzymes of rat stomach mucosa

Abstract

1. 1. Rat stomach mucosa exhibited three distinguishable phospholipid-deacylating enzyme activities: lysophospholipase, phospholipase A 1 and phospholipase A 2. 2. 2. The lysophospholipase hydrolyzed 1-palmitoyl lysophosphatidylcholine to free fatty acid and glycerophosphorylcholine. 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. 3. 3. Phospholipase A 2 and phospholipase A 1 deacylated dipalmitoyl phophatidylcholine 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. 4. 4. In the absence of bile salts, phospholipase A 1 exhibited pH optima at 6.5 and 9.5 and phospholipase A 2 at pH 6.5, 8.0 and 9.5. The pH optima for phospholipase A 1 were shifted to pH 3.0, 6.0 and 9.0 in presence of sodium taurocholate; the activity was detected only at a single pH of 9.5 in the presence of sodium deoxycholate and at pH 10.0 in the presence of sodium glycocholate. Phospholipase A 2 optimum activity was displayed at pH 3.0, 6.0 and 8.0 in presence of taurocholate, pH 7.5 and 9.0, in presence of glycocholate and only at pH 9.0 in presence of deoxycholate. 5. 5. Ca 2+ was essential for optimum activity of phospholipases A 1 and A 2. But phospholipase A 1 lost complete activity in presence of 0.5 mM ethyleneglycolbis-(β-aminoethylether)-N,N'-tetraacetic acid (EGTA) at pH 6.0, whereas phospholipase A 2 lost only 50%. 6. 6. Phospholipases A 1 and A 2 retained about 50% of their activities by heating at 75° for 10 min. At 100°, phospholipase A 1 retained 22% of its activity, whereas phospholipase A 2 retained only 7%.