Hydrolysis of dipalmitoylphosphatidylcholine small unilamellar vesicles by porcine pancreatic phospholipase A₂

The hydrolysis of small unilamellar vesicles made of dipalmitoylphosphatidylcholine by pancreatic phospholipase A₂ has been studied under various conditions of temperature and enzyme and substrate concentration using the following three different experimental protocols. 1) When the enzyme was added to the substrate vesicles after being separately adjusted to the temperature of the experiments hydrolysis occurred instantaneously only in the temperature range where the lipid is known to exist in its gel phase, while above the transition range no hydrolysis occurred. Within the transition range, the time course of hydrolysis was characterized by an initial very slow rate of hydrolysis (latency phase) followed by an abrupt increase in the rate after a time τ, which is a complex function of temperature and enzyme to substrate ratio. 2) When an enzyme-substrate mixture was first preincubated below T(m) and then temperature jumped to a temperature above or within the transition range, the latency phase was markedly shortened. When the temperature jump was to the transition range, this effect is observed even if Ca²⁺ is absent in the preincubation mixture. However, instantaneous hydrolysis was observed upon temperature jumping the mixture to a temperature high above T(m) only if Ca²⁺ was present in the preincubation medium. 3) In temperature scanning experiments, hydrolysis was followed while changing the temperature of the enzyme-substrate mixture continuously. Heating an enzyme-substrate mixture from room temperature resulted in an abrupt onset of hydrolysis when the transition range was approached. These results lead us to conclude that two distinctly different steps precede rapid hydrolysis of dipalmitoylphosphatidylcholine small unilamellar vesicles by pancreatic phospholipase A₂: a Ca²⁺-independent binding of the enzyme to the substrate vesicles, which for chemically pure bilayers occurs best in the gel phase. This step is followed by a Ca²⁺-dependent activation of the initially formed enzyme-substrate complex. The latter step only occurs under conditions where the bilayer possesses packing irregularities and probably involves a reorganization of the enzyme-substrate complex. At least one of these two steps appears to involve enzyme-enzyme interaction.