Activation of sphingomyelinase in lipid monolayer is related to interfacial water activity

Recently it has been hypothesized that surface pressure of a lipid monolayer is a direct measure of thermodynamic activity of interfacial water and therefore surface pressure-dependent processes in lipid bilayers and monolayers are modulated essentially by interfacial water activity [S. Damodaran, Colloids Surf. B: Biointerf. 11 (1998) 231; C.S. Rao, S. Damodaran, Colloids Surf. B: Biointerf. 34 (2004) 197]. If the hypothesis is true, then it should be a general one and ought to be system independent. To further test this hypothesis, the specific activity of sphingomyelinase (SMase) was studied in two disparate systems, one involving sphingomyelin (SM) monolayer at various surface pressures at the air-water interface and the other involving a solid-state SMase-SM 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 (ERH). In both these physically different systems, the dependence of specific activity of SMase on ERH and aws was very similar. In both cases, the specific activity exhibited a maximum at ERH (or aws) ∼0.3, which suggested that the apparent surface pressure-dependence of interfacial activation of lipolytic enzymes might be actually related to modulation of the hydration state of the enzyme through the control of thermodynamic activity of water in the lipid-water interfacial region.