Inactivation of the human thyroid peroxidase by ultrasound cavitation]

The inactivation kinetics of a human thyroid peroxidase protein fraction upon sonication (ultrasound frequency 27 kHz, power 60 W/cm2) of the enzyme solution in 15 mM phosphate buffer, pH 7.5, was studied. To quantitatively characterize the dependence of the slowest stage of the human thyroid peroxidase inactivation on temperature (36.0-50.4) degrees C, an effective constant of ultrasound inactivation rate Kin(US) was used. From the temperature dependence of Kin(US) at temperatures below 43 degrees C, the activation energy was estimated to be 8.11 kcal/mol. It was shown that the rate of human thyroid peroxidase inactivation strongly depends on the concentration of total protein in solution: the kin(US) value decreases more than sixfold in the protein concentration range from 0.2 to 0.8 mg/ml. It was also shown that poly(2-aminodisulfide-4-nitrophenol), its complexes with human serum albumin as well as the complexes human serum albumin--poly(gallic acid disulfide) substantially inhibit the ultrasound-induced inactivation of the enzyme and can be its effective stabilizers in the ultrasound cavitation field. This confirms the suggestion that active free radicals HO., O2.- and HO2. play a key role in the inactivation of human thyroid peroxidase. A general scheme of the inactivation of human thyroid peroxidase is proposed, which represents a chain of successive and parallel reversible and irreversible elementary steps.