Lipocortin inhibition of extracellular and intracellular phospholipases A2 is substrate concentration dependent

Hydrolysis of Escherichia coli membrane phospholipids by pancreatic phospholipase A2 was inhibited by lipocortin from human monocytes in a substrate dependent manner. Inhibition was completely overcome at substrate concentrations above 250 microM. Lipocortin also inhibited partially purified preparations of two intracellular phospholipases A2 isolated from rat liver mitochondria and rat platelets when these enzymes were assayed at low micromolar concentrations of phosphatidylethanolamine. Inhibition gradually decreased with increasing substrate concentrations both for pancreatic and platelet phospholipase A2 and became completely abolished above 15 and 50 microM phosphatidylethanolamine, respectively.