Effects of conventional heating (CH) and microwave (MW) on the structure and activity of horseradish peroxidase (HRP) in buffer solution were studied. CH incubation between 30 and 45 degrees C increased activity of HRP, reaching 170% of residual activity (RA) after 4-6 h at 45 degrees C. CH treatment at 50 and 60 degrees C caused HRP inactivation: RA was 5.7 and 16.7% after 12 h, respectively. Secondary and tertiary HRP structural changes were analyzed by circular dichroisrn (CD) and intrinsic fluorescence emission, respectively. Under CH, activation of the enzyme was attributed to conformational changes in secondary and tertiary structures. MW treatment had significant effects on the residual activity of HRP. MW treatment at 45 degrees C/30 W followed by CH treatment 45 degrees C regenerated the enzyme activity. The greatest loss in activity occurred at 60 degrees C/60 W/30 min (RA 16.9%); without recovery of the original activity. The inactivation of MW-treated HRP was related to the loss of tertiary structure, indicating changes around the tryptophan environment. (C) 2014 Elsevier Inc. All rights reserved.