Lipase activity on lipid/polymer binary monolayers: Lateral diffusion-controlled enzyme kinetics

This is a follow-up study to our recent reports of lipase catalysis on uniphasic binary monolayers of L-α-dilauroylphosphatidylcholine (DLPC) and cholesterol. The earlier studies were undertaken to show that the reaction kinetics on monolayers of ester hydrolysis catalyzed by an interface active enzyme, lipase, is closely correlated to the lateral dynamics, i.e., the hydrolysis of a lipase substrate is a diffusion-controlled process, whereby the rates were quantitatively analyzed in terms of a theory of purely two-dimensional diffusion-controlled reactions. In so establishing, cholesterol was used as the second component to retard the lateral diffusion coefficient of DLPC to different extents. On the other hand, phospholipids and cholesterol are being found to form "condensed complexes" even in homogeneous monolayers, hence the finding of the diffusion-controlled processes might well be construed to be unique to the phospholipid-cholesterol system. To show that the diffusion-controlled reaction kinetics on monolayers is of more general validity, i.e., not system-specific, we chose a completely different second component to constitute a binary homogeneous monolayer system by which we can vary the lipid diffusion coefficient. The second component is a synthetic polymer, poly(tert-butyl methacrylate) (PtBMA), well-known for its surface activity on the air/water interface, and is demonstrated to form uniphasic monolayers with DLPC at the air/aqueous buffer (pH 7) interface. Upon completing the entire set of measurements for the diffusion coefficients and the hydrolysis rates with the same substrate, umbelliferone stearate, we reached the conclusions that what we found with DLPC/ cholesterol are not unique to the system but equally applicable to DLPC/PtBMA, whereas the polymer is not likely to exert the same sort of retardation effect on monolayer dynamics by specific interactions that result in condensed complexes of phospholipid and cholesterol.