A number of peroxidases, such as lignin peroxidase and manganese peroxidase have proved to be useful for industrial applications. Some studies on the effects of temperature and pH stability have been carried out. It is known that veratryl alcohol increases their stability in the range 28-50 degrees C and is oxidized, leading to veratryl aldehyde formation. Similar results with horseradish peroxidase (HRP) in the presence of cofactors were found, but the oxidation of veratryl alcohol in the absence of cofactors was extremely labile at acid pH and inactivated in a few minutes. Considering the growing industrial application of HRP, knowledge of its stability and denaturation kinetics is required. In this study, horseradish peroxidase pool (HRP-VI) and its isoenzymes HRP-VIII (acid) and HRP-IX (basic) have been shown to catalyze the oxidation of veratryl alcohol to veratryl aldehyde in the presence of hydrogen peroxide at pH 5.8 in the 35-45 degrees C range and in the absence of any cofactors. Heat and pH denaturation experiments in the presence and absence of veratryl alcohol incubation were conducted with HRP-VI and HRP-IX isoenzymes. HRP-IX was the most active isoenzyme acting on veratryl alcohol but HRP-VI was the most stable for the temperature range tested. At 35 degrees C the HRP pool presented decay constant (Kd) values of 5.5 x 10(-2) h(-1) and 1.4 10(-2) h(-1) in the absence and presence of veratryl alcohol, respectively, with an effective ratio of 3.9. These results present a new feature of peroxidases that opens one more interesting application of HRP to industrial processes.84279-86