The activation of porcine pancreatic phospholipase A2 by dipalmitoylphosphatidylcholine large unilamellar vesicles. Analysis of the state of aggregation of the activated enzyme.

Previous work from this laboratory and others has shown that the hydrolysis of pure dipalmitoylphosphatidylcholine (DPPC) liposomes by porcine pancreatic phospholipase A2 in the vicinity of the gel-to-liquid crystal phase transition is characterized by a slow initial phase followed by an apparent burst of activity. In this article we report a detailed quantitative analysis of the early time course of the hydrolysis of dipalmitoylphosphatidylcholine large unilamellar vesicles at 38 degrees C. Several kinetic models to quantitatively describe the data were considered. The most conservative model consistent with the kinetic data is one in which the enzyme initially binds the bilayer and becomes activated via a process that requires the formation of protein dimers on the surface of the membrane. The relevant kinetic parameters of the model are reported.