Dependence on neutral salt concentration of the latency phase in the time course of hydrolysis of dimyristoylphosphatidylcholine liposomes by phospholipase A2

The time course of the hydrolytic action of porcine pancreatic phospholipase A2 on sonicated dimyristoylphosphatidylcholine liposomes in the presence of variable NaCl concentrations has been studied at temperatures between 17 and 36 °C; at these temperatures liposomes are in the gel phase. At a NaCl concentration of 10 mM, the hydrolysis shows a small and constant lag period of 6–8 min at all temperatures within this range. As the temperature is raised into the liquid crystalline range, the latency phase lengthens monotonically so that at 36 °C it reaches 55 min. An increase in the NaCl concentration to 1 M makes the lag period longer at all temperatures studied, with the exception of the phase transition range (near 24 °C); within this temperature range, a small reduction in the lag time is observed. The increase in the length of the latency period at high salt concentrations may be due to screening of the negative surface charge generated by the nascent fatty acid which seems to be essential for the efficient interfacial binding of the enzyme. In the phase transition range of the lamellae, the unfavorable effect of high salt concentrations on the electrostatic binding of the enzyme appears to be overcome by another type of interaction. Recent findings raise the possibility that this interaction could be hydrophobic in nature.Key words: phospholipase A2, liposomes, hydrolysis, salt, latency phase.