Relationship between the activity of pancreatic phospholipase A2 and the physical state of the phospholipid substrate.

When pancreatic phospholipase A2 is mixed with small unilamellar vesicles made from dipalmitoyl phosphatidylcholine at or above the phase transition temperature (tm), the time course of hydrolysis exhibits a distinct lag period. On the other hand, if the enzyme is preincubated with substrate for a short period of time below the transition temperature (during which time only limited hydrolysis occurs) and then assayed at high temperature, no lag period is observed. This phenomenon does not appear to be the result of product formation. Instead, the reported results lend substantial support to the hypothesis that a relatively slow substrate-enzyme organizational step is required as part of an activation process and this is most rapid when the substrate is in the gel state. However, the intrinsic activity (after the initial step of activation) is maximal when the substrate is in the liquid-crystalline state.