The action of purified lysophospholipases on microsomal membrane-bound lysophosphatidylcholine

Rat liver membranes were labelled by intraperitoneal injection of [Me-14C]choline chloride. Isolated microsomal membranes were briefly treated with pancreatic phospholipase A2 to produce different levels of membrane-bound lysophosphatidylcholine. The hydrolysis of this lysophosphatidylcholine by two purified lysophospholipases from beef liver was studied. The specific activity of enzyme I at saturating membrane concentrations appeared to increase linearly with the lysophosphatidylcholine level in the membranes until the lysoderivative represented 15% of the original phosphatidylcholine. In contrast, the specific activity of enzyme II was independent of the lysophosphatidylcholine level, at least in the range of 4.9-34.0% tested. These different kinetics are discussed in terms of the possible functions of both enzymes in liver.