Non-proteolytic solubilization of bovine thyroid peroxidase: thermodynamic parameters of the thermoinactivation

1. A subcellular fractionation from bovine thyroid gland homogenate was carried out by differential centrifugation. The maximal peroxidase activity was found in the microsomal fraction. 2. The effect of non-proteolytic agents on the solubilization of the enzyme was studied. It was found that non-ionic detergents with Hydrophilic Lipophilic Balance (HLB) between 12 and 14, such as Triton X-100 and Brij 96, were the best solubilizing agents. 3. The effect of pH, ionic strength, time and temperature of incubation of the peroxidase solubilization was analyzed. 4. The thermodynamic parameters of the thermoinactivation of solubilized enzyme and the activation energy (Ea) of the peroxidase-catalyzed reaction were calculated. 5. The values obtained for these parameters were very similar to those of the plant peroxidases, suggesting a common catalytic mechanism, as Bardsley et al., Eur. J. Biochem. (1982) have pointed out. 6. However thyroid peroxidase does not undergo any reactivation as plant peroxidases do, due to the role of the hydrophobic interactions which hold the peroxidase bound to microsomal membranes in the thyroid glands.