Origin of the latency phase during the action of phospholipase A2 on unmodified phosphatidylcholine vesicles.

ABSTRACT

The reaction progress curve for the action of pig-pancreatic phospholipase A2 on dimyristoylphosphatidylcholine vesicles is characterized under a variety of conditions. The factors that regulate the rate of hydrolysis during the presteady-state phase determine the latency period. The results demonstrate that the accelerated hydrolysis following the latency phase of the reaction progress curve is due to the product-assisted binding of the enzyme to the substrate bilayer by chaning the number of bindings sites and therefore the binding equilibrium. A critical mole fraction of products appears to be formed in the substrate bilayers before the steady-state phase of hydrolysis begins. The latency phase shows a minimum at the phase-transition temperature of the substrate vesicles; however, we did not observe a significant binding of the enzyme to pure substrate bilayers even at the phase-transition temperature. The rate of binding of the enzyme is found to be fast and the rate of desorption of the bound enzyme is very slow compared to the latency phase. The rate of redistribution of products between substrate bilayers is rather slow. These observations demonstrate that during the latency phase of the action of phospholipase A2, a critical mole fraction of products is formed in the substrate bilayer.