Product Accumulation during the Lag Phase as the Basis for the Activation of Phospholipase A2 on Monolayers

Abstract

Interfacial activation of pig pancreatic phospholipase A2 (PLA2) in the steady-state phase, which follows a lag period in the reaction progress at the air/water interface, is shown to be due to the accumulation of the products of hydrolysis. The kinetic and equilibrium results clearly show that the products accumulated during the lag leads to a higher apparent rate of hydrolysis of didecanoylphosphatidylcholine (PC10), and the lag virtually disappears if the products are already present. Our results invalidate the assumption that all the products formed at the air−water interface rapidly leave the monolayer. PC10 forms laterally compressible insoluble monolayer, whereas the products of hydrolysis do not form such a stable monolayer. Surprisingly, however, a significant amount of the product is retained in the PC10 monolayer at equilibrium, as well as under the steady-state condition of the reaction progress. The excess product, formed in situ or cospread with PC10, leaves the monolayer slowly with a half-...