A model for the activation of porcine pancreatic phospholipase A2 for the hydrolysis of dipalmitoylphosphatidylcholine liposomes

D. Lichtenberg, M. Menashe, R. L. Biltonen

Research output: Contribution to journalArticlepeer-review

Abstract

The hydrolysis of large unilamellar vesicles made of dipalmitoylphosphatidylcholine (DPPC) by porcine pancreatic phospholipase A2 (PLA2) have been studied as a function of temperature. The time courses of these reactions depend on the structural state of the phospholipid substrate. In neither the gel phase nor the liquid-crystalline phase, hydrolysis occurs, whereas in the phase transition region (39-42°C) the hydrolysis is characterized by a phase of slow hydrolysis (latency phase of a length of time τ), followed by an abrupt increase in the rate of hydrolysis. The latency (τ) is minimal at the phase transition temperature (Tm) in agreement with previous data, which suggested that activation requires the existence of packing irregularities. Such irregularities exist only in the transition region and maximize at the Tm. Preincubation of a mixture of enzyme and substrate vesicles at a temperature below Tm, when followed by dilution into a medium at the transition region, results in virtually instantaneous hydrolysis. This hydrolysis is independent of the presence of Ca2+ in the preincubation medium. These results are interpreted in terms of the following scheme for the hydrolysis of DPPC by PLA2: First, the enzyme binds to the lipid bilayer in a Ca2+-independent step which occurs best for gel-phase lipids. This step is then followed by activation of the initially formed enzyme-substrate complex, which requires Ca2+ and can only occur if packing irregularities exist in the membrane (in the thermal transition range of the vesicles). Reaction products play a role in the hydrolysis at the Tm range by increasing the binding of PLA2 to the vesicle surface. In this temperature range, increased binding is sufficient to cause virtually instantaneous hydrolysis, since activation is relatively fast.

Original languageEnglish
Pages (from-to)293-301
Number of pages9
JournalColloids and Surfaces
Volume14
Issue number3-4
DOIs
StatePublished - Jun 1985

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