Influence of pH and high pressure on the thermal inactivation kinetics of horseradish peroxidase

The inactivation kinetics of horseradish peroxidase were determined at seve ral values of pH (3, 4, 7, 9 and 10), temperature (40 to 70 degrees C) and pressure (7, 8 and 9 kbar). The data were obtained by taking samples from t he moment that pressure inside the hydrostatic high pressure vessels reache d the value specified, and therefore relate to isothermal and isobaric cond itions. The inactivation kinetics were non-log linear and could be individu ally well described by the conventional two-fraction model, but the overall consistency of the kinetic parameters of this model was poor and the model was clearly not robust. Inversely, the kinetic model based on the Weibull probability distribution function yielded good individual fits and also a g ood overall consistency of the kinetic parameters. The whole set of 327 dat a points could be well described by an overall model considering that the s hape parameter (beta) varied with pressure but not with temperature and tha t the rate parameter (1/alpha) varied with temperature according to an Arrh enius law, with a pressure-independent activation energy, and varied expone ntially with pressure. The activation energy and the pressure-sensitivity p arameter did not vary with pH up to pH 9. The results suggested a discontin uity of the kinetic behaviour at pH 10, with a different inactivation mecha nism prevailing.