Cosolvent effects on gel-entrapped oxidoreductase: the glucose oxidase model.

Abstract

An intrinsic problem often involved in biotransformations carried out by immobilized cells is the poor solubility of substrate and product in water. Increase in hydrophobic substrate availability to such gel-entrapped cells may be attained by the replacement of a fraction of the aqueous medium by water-miscible solvents (cosolvents). The introduction of cosolvents results in increased solubility, but may simultaneously affect enzymic activity and stability. Recently, criteria and guidelines for cosolvent selection on the basis of its effect on intracellular enzyme stability were reported (Freeman, A., and Lilly, M.D. (1987) Appl. Microbiol. Biotechnol. 25, 495-501). In order to understand the impact of the preferable or unsuitable cosolvents on enzyme kinetics and stability, the effects of 1-5 M concentrations of a series of cosolvents (e.g., ethylene glycol, dimethylsulfoxide, N,N-dimethylformamide, ethanol) on a well-characterized, highly specific enzyme model (glucose oxidase) were investigated. The presence of 1-5 M of the cosolvents studied imposed 10-50% reduction in Vmax of the enzyme, but Km was only mildly affected (+/- 25%). This inhibition was attributed to cosolvent effect on small, reversible, conformational changes in the enzyme native structure. Determination of the rate constant of thermal inactivation (at 55 degrees C) of glucose oxidase, in the presence of cosolvents, was employed for the quantitative evaluation of cosolvent effect on enzyme stability. A clear pattern of cosolvent preference in respect to its denaturing effect was obtained, which was identical to the pattern previously observed in a study of oxidoreductases operating from within a whole cell. In both cases diols (e.g., ethylene glycol) were found to be the preferable group of cosolvents. Our results indicate that a soluble enzyme and an intracellular enzyme operating from a whole cell are affected by cosolvents via the same mechanism.