Catalytic activity and stability of xanthine oxidase in aqueous-organic mixtures

In the present study, bovine milk xanthine oxidase activity in various aqueous-organic mixtures and the effects of pH, temperature, and lyophilization on the enzyme activity have been investigated. The enzyme was incubated with xan-thine as the substrate in Sorenson's phosphate buffer (pH 7.0) containing 0.1 mM EDTA, and the activity was determined spectrophotometrically in the absence and presence of different fractions of nine water-miscible organic solvents at 27-50°C and at different pH values ranging from 6 to 9. The organic solvents reduced the enzyme activity to different extents. In spite of these inhibitory effects, the enzyme showed relatively good stability in the aqueous-organic mixtures compared with the aqueous medium. A significant increase in the activity of the lyophilized enzyme was observed in pure organic solvents.