Recognition of different pools of phosphatidylglycerol in intact cells and isolated membranes of Acholeplasma laidlawii by phospholipase A2

Abstract

Phospholipase A2 (EC 3.1.1.4) from pig pancreas hydrolyzes phosphatidylglycerol in intact cells and isolated membranes of A. laidlawii. Complete degradation of phosphatidylglycerol in intact cells at 37.degree. C does not result in lysis as shown by the retention of intracellular K+ and the cytoplasmic G-6-P, and inability to detect activity of membrane-bound intracellular NADH-oxidase. A. laidlawii was grown on linoleic acid. Phospholipase A2 treatment of these cells at 5.degree. C, at which temperature the lipids are still in the liquid-crystalline state, results in a rapid breakdown of 50% of the phosphatidylglycerol. The residual phosphatidylglycerol can be hydrolyzed only at elevated temperatures and at much smaller rates, depending strongly on the incubation temperature. When membranes isolated from these cells are incubated at 5.degree. C, 70% of the phosphatidylglycerol is hydrolyzed immediately. The hydrolysis of the residual 30% is again strongly temperature dependent. Cells were grown on palmitate, elaidate or oleate to investigate possible effects of the lipid phase transition on the accessibility of phosphatidylglycerol for phospholipase A2. Under conditions in which all the lipid is in the solid state, no hydrolysis occurs. When solid and liquid-crystalline lipid phases coexist, a limited hydrolysis of phosphatidylglycerol can be observed. The disposition of phosphatidylglycerol in 3 different pools in the membrane of A. laidlawii is demonstrated. Phospholipase A2 was used to discriminate between these pools and to estimate the amount of phosphatidylglycerol which is present in the liquid-crystalline phase. The present data do not allow a definite localization of the phosphatidylglycerol pools.