Analysis of the kinetic patterns of horseradish peroxidase thermal inactivation in sodium phosphate buffer solutions of different ionic strength

The thermal inactivation of horseradish peroxidase was studied in sodium phosphate buffer solutions and in pure water at pH 7 in the temperature range of 70-95°C. The sodium phosphate ions concentration affected both the thermostability and the kinetic patterns and had a stabilizing effect. The gradual change observed at low concentrations made a series-type mechanism theoretically more coherent with the experimental observations than the conventionally applied two-fraction model. In water the kinetics is apparently first order at high temperatures, while the results obtained at 25°C support the occurrence of a series-type inactivation mechanism. The pH and enzyme concentration also affect the inactivation profile, supporting the conclusion that the thermal inactivation is not a monomolecular process with respect to protein concentration.