Detergentless microemulsions as media for enzymatic reactions. Cholesterol oxidation catalyzed by cholesterol oxidase

Catalytic acitivity and stability of cholesterol oxidase dissolved in ternary systems composed on n-hexane, isopropanol, and water were studied. The dependence of catalytic acitivity on the composition of the system revealed two maxima, in contrast to the behaviour of previously studied enzymes where a single maximum has been observed. The stability profile of cholesterol oxidase showed a single sharp maximum coinciding with the microemulsion region of the phase diagram. Both catalytic activity and the first-order inactivation rate constant of cholesterol oxidase dissolved in n-hexane/isopropanol/water ternary systems were found to decrease with decreasing temperature. This decrease was more rapid for the inactivation rate constant than for catalytic activity, the activation energies being 200 and 60 kJ mol−1, respectively. Preparative conversion of cholesterol to cholestenone catalyzed by cholesteroloxidase in n-hexane/isopropanol/water ternary systems was carried out with 100% yield. Decreased temperature and the presence of catalase were required to achieve high degrees of cholesterol conversion. A simple procedure suitable for rapid separation of the reaction product and recovery of the enzyme was developed.