Acyl chain specificity and kinetic properties of phospholipase A1 and A2 of bone marrow-derived macrophages.

The fatty acyl specificity of phospholipase A1 and A2 in homogenates of mouse bone marrow-derived macrophages was determined using phosphatidylcholine and phosphatidylethanolamine of different acyl chain composition. Phosphatidylcholine with arachidonoyl at position 2 was cleaved preferentially by an alkaline phospholipase A2 (pH-optimum 9.0) leading to selective liberation of arachidonic acid. In contrast, phosphatidylcholines with oleoyl or linoleoyl at position 2 were degraded mainly by an acid phospholipase A1 (pH-optimum 4-5) resulting in a conservation of these fatty acids esterified in lysophosphatides. Substrate kinetics of the alkaline phospholipase A2 revealed a 30 fold higher affinity (Km = 3.8 X 10(-7) M) for 1-acyl-2-arachidonoyl-glycerophosphocholine compared to 1-acyl-2-oleoyl-glycerophosphocholine. The kinetic data were not influenced by endogenous lipids indicating that exogenous substrates do not equilibrate with cellular lipids. These results are suitable to explain a selective liberation of arachidonic acid from a mixture of phospholipids.