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Catalytic Activity and Stability of Xanthine Oxidase in Aqueous–Organic Mixtures

M. R. Rashidi1*, M. H. Soruraddin2, F. Taherzadeh2, and A. Jouyban3

1Biotechnology Research Center, Tabriz University of Medical Sciences, Tabriz 51664-14766, Iran; E-mail: rashidi@tbzmed.ac.ir

2Analytical Chemistry Department, School of Chemistry, Tabriz University, Tabriz 51664-14766, Iran

3Pharmaceutical Chemistry Department, School of Pharmacy, Tabriz University of Medical Sciences, Tabriz 51664-14766, Iran

* To whom correspondence should be addressed.

Received December 11, 2007; Revision received July 28, 2008
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 xanthine 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.
KEY WORDS: xanthine oxidase, aqueous–organic mixtures, enzyme activity, thermal stability, lyophilization

DOI: 10.1134/S0006297909010155