A study of entrapped enzyme stability and substrate diffusion in a monoglyceride-based cubic liquid crystalline phase

Our recent results have shown that enzymes with molecular weights of up to 590 kDa can be entrapped in cubic liquid crystalline phases in lipid/aqueous systems. In the present study, both pure monoolein and monoolein/phosphatidylcholine mixtures were used for the preparation of the cubic phases. Electrochemical measurements of the enzyme activity show that the entrapment in the cubic phase is liable to stabilise the enzyme. The interactions between protein molecules and a periodically curved lipid bilayer in these systems still remain to be elucidated. However, our data show that the composition of the lipid might influence the stability of the enzyme, that is the introduction of the zwitterionic phosphatidylcholine leads to an increase in the long-term stability of glucose oxidase. This can probably be assigned both to the differences in the polar interface of the lipid bilayer and the changes in structure of the cubic phase. The properties of biosensors constructed from cubic phases containing glucose oxidase and ceruloplasmin were compared. Both enzymes have about the same molecular weight, but different electrochemical reactions were used for monitoring the enzyme activity. We have also studied the diffusion of a substrate molecule, glucose, in the cubic phase by means of holographic laser interferometry, nuclear magnetic resonance (NMR) and chronoamperometry to obtain more information on the cubic phase as a support for enzyme immobilisation.