Phospholipase A2 in rat-lung microsomes: Substrate specificity towards endogenous phosphatidylcholines

Abstract

1. 1. Isolated rat lungs were perfused with a variety of radioactive precursors to label the phosphatidylcholines of the microsomal and lamellar body fractions. These endogenously labelled phosphatidylcholines were used as substrates in experiments to identify and characterize phospholipase A activity in lung subcellular fractions.

2. 2. The microsomal fraction was found to contain a phospholipase A specific for the 2-position of endogenous phosphatidylcholines. The enzyme operated optimally at pH 8.5 and required 10 mM Ca2+ for maximal activity.

3. 3. No evidence was found for the existence of phospholipase A activity in lamellar bodies.

4. 4. The microsomal phospholipase A2 was more active towards phosphatidyl-cholines containing an unsaturated fatty acid at the 2-position than towards the disaturated phosphatidylcholines.

5. 5. Microsomal disaturated phosphatidylcholines labelled with [1-14C]acetate (endogenously synthesized palmitate) were hydrolysed by the microsomal phospholipase A2; however, no hydrolysis occurred when [9,10-3H2]palmitate was used as a precursor notwithstanding the fact that the label from [1-14C]-acetate is mainly incorporated into the 1-position and that from [9,10-3H2]-palmitate almost exclusively into the 2-position of the disaturated phosphatidylcholines of rat-lung microsomes.

6. 6. These results suggest the existence of two pools of disaturated phosphatidylcholines in rat-lung microsomes. They are consistent with the concept that dipalmitoylphosphatidylcholine synthesized by remodeling of unsaturated phosphatidylcholines with exogenously supplied palmitic acid, is not hydrolysed by phospholipase A2 of lung microsomes.