Surface pressure dependence of phospholipase A2 activity in lipid monolayers is linked to interfacial water activity

The specific activity of pancreatic phospholipase A2 (PLA2) was studied in two disparate systems, one involving phosphatidylcholine monolayer at various surface pressures at the air-water interface and the other involving a solid-state system exposed to various equilibrium relative humidity (ERH). The results were examined in terms of thermodynamic activity of water in the interfacial region (aws*) and in the hydrated solid phase (aw). In both these physically different systems, the specific activity versus aw and aws* profiles of PLA2 were remarkably similar. In both cases, the specific activity exhibited a maximum at aw (or aws*) approximately 0.3. These results suggested that the mechanism of control of PLA2 activity at the lipid-water interface might involve modulation of the hydration state of the enzyme through control of the thermodynamic activity of water in the interfacial region. Extension of these results to biomembranes suggests that one of the functions of lipid bilayer might be the control of local water activity at the lipid-water interface. In biological membranes, localized subtle changes in interfacial water activity may occur as a result of local stretching or compression of the membrane facilitated by conformational changes in membrane-bound receptor proteins.