Is Interfacial Activation of Lipases in Lipid Monolayers Related to Thermodynamic Activity of Interfacial Water?

Interfacial activation of lipases from Candida rugosa (CRLs) and porcine pancreas (PPLs) was studied by simultaneously monitoring the rate of accumulation of the lipase in the lipid monolayer and the rate of hydrolysis of the dicaprin monolayer. The rate of accumulation of ¹⁴C-labeled lipase in the lipid monolayer was determined using a surface radiotracer technique, and the rate of hydrolysis of dicaprin at a constant surface pressure was measured using a zero-order trough. The specific activity-time profiles of these lipases in the lipid monolayer exhibited a maximum. This maximum in the specific activity profile was due to two molecular processes at the interface: One involved activation of the enzyme from an inactive configuration to an active configuration, and the other involved subsequent inactivation and/or inhibition of the enzyme. The results suggested that, upon adsorption to the interface, the enzyme progressively undergoes conformational changes from an inactive to active state and then to an inactive state. It appeared that, at least for PPLs and CRLs, the active state (E*) was not the final state of the enzyme at the interface, but only a transient state. The maximum in the specific activity-time profile was high at higher surface pressures, indicating that the rates of activation and inactivation of the enzymes were surface-pressure dependent. Because the surface pressure is related to the thermodynamic activity of interfacial water, the surface pressure dependency of the lipase activity was analyzed in terms of the interfacial water activity. A kinetic model based on these new findings was also proposed.