Kinetic theory of enzymatic reactions in reversed micellar systems : application of the pseudophase approach for partitioning substrates

A kinetic theory is proposed for enzymatic reactions proceeding in reversed micellar systems in organic solvents, and involving substrates capable of partitioning among all pseudophases of the micellar system i.e. aqueous cores of reversed micelles, micellar membranes and organic solvent. The theory permits determination of true (i.e. with reference to the aqueous phase, where solubilized enzyme is localized) catalytic parameters of the enzyme, provided partition coefficients of the substrate between different phases are known. The validity of the kinetic theory was verified by the example of oxidation of aliphatic alcohols catalyzed by horse liver alcohol dehydrogenase in the system of reversed sodium bis(2-ethylhexyl)sulfosuccinate (AOT, aerosol OT) micelles in octane. In order to determine partition coefficients of alcohols between phases of the micellar system, flow microcalorimetry technique was used. It was shown that in the first approximation, the partition coefficient of the substrate in a simple biphasic system consisting of water and corresponding organic solvent can be used as an estimate for the partition coefficient of the substrate between aqueous and organic solvent phases of the micellar system. True values of the Michaelis constant of alcohols in the micellar system, determined using suggested approach, are equal to those obtained in aqueous solution and differ from apparent values referred to the total volume of the system. The results clearly show that the previously reported shift in the substrate specificity of HLADH, observed on changing from aqueous solution to the system of reversed aerosol OT micelles in octane, is apparent and can be explained on the basis of partitioning effects of alcoholic substrates between phases of the micellar system.