Adsorbed and spread films of bovine milk xanthine oxidase at the air-water interface

The properties of adsorbed and spread films of bovine milk xanthine oxidase (XO) at the air-water interface were investigated by the film balance technique. Native XO from solution, as opposed to a freeze-dried preparation, could not be spread quantitatively under the conditions used in this study. The surface pressure (Pi) versus area (A) isotherms indicated a smaller area per molecule at pH 5.0, compared with at pH 7.4 and 10.0. Adsorption of XO at the air-water interface showed a strong dependence on the concentration. At all pH values studied, the rates of adsorption increased with the ionic strength. The plateau level of Pi at pH 10.0 was lower and the spread films were more expanded than at pH 5.0 and 7.4. This indicates a different conformation of XO at this pH. The adsorption rate (d Gamma/dt) of XO was determined for different subphase compositions and the highest adsorption rate was observed at pH 5.0, with the lowest rate at pH 7.4. No significant difference in the interfacial properties between the dehydrogenase and the oxidase form of XO could be established. (C) 1998 Elsevier Science B.V. All rights reserved.