Recognition of acidic phospholipase A2 activity in plasma membranes of resident peritoneal macrophages

Phospholipase (PLase) activities in the plasma membrane of guinea pig peritoneal macrophages were studied, as these enzymes having such activity may be candidates for the release of arachidonic acid (AA) from phosphatidylcholine (PC). An AA release system operating at acidic pH was identified in the macrophage plasma membrane and characterized. This membrane-bound acidic PLase A2 had an optimum pH at 4.5, and enzyme activation was observed in Ca++-free medium; but the maximum activity was found at 0.5 mM Ca++ concentration. The Km value for PC of acidic PLase A2 was 4.2 microM, and a Michaelis-Menten relationship was evident. Calcium might act as a cofactor at some intermediate step during the activation of acidic PLase A2 in light of the uncompetitive manner of Ca++ action. Furthermore, the release of [3H]-AA from preradiolabelled macrophage plasma membranes occurred with the addition of Ca++ at pH 4.5. These data suggest that the acid PLase A2 is a component of the plasma membrane and is not due to lysosomal contamination since membrane-bound acidic PLase A2 properties are opposite to those found for lysosomal PLase A2.