The effect of high hydrostatic pressure (HHP) on the kinetics of thermal inactivation of xanthine oxidase (XOx) from bovine milk was Studied. Inactivation of XOx followed pseudo-first-order kinetics at 0.1-300 MPa and 55.0-70.0 degrees C. High pressure up to at least 300 MPa stabilized XOx at all the studied temperatures. The highest stabilization effect of HHP on XOx was at 200-300 MPa at 55.0 and 58.6 degrees C, and at 250-300 MPa at 62.3-70.0 degrees C. The stability of XOx increased 9.5 times at 300 MPa and 70.0 degrees C compared to atmospheric pressure at the same temperature. The activation energy of inactivation of XOx decreased with pressure and was 1.9 times less at 300 MPa (97.0 +/- 8.2 kJ mol(-1)) than at 0.1 MPa (181.7 +/- 12.1 kJ mo1(-1)). High pressure decreased the dependence of the rate constant of inactivation to temperature effects compared to atmospheric pressure. The stabilizing effect of HHP on XOx was highest at 70.0 degrees C where the activation volume of inactivation of XOx was 28.9 2.9 crn(3) mor(-1). A second approach to try to increase XOx stability involved hydrophobic modification using aniline or benzoate. However, the thermal stability of XOx remained unaffected after 8-14 modifications of carboxyl side groups per XOx monomer with aniline, or 12-17 modifications of amino side groups per XOx monomer with benzoate.