Degradation of Thyroxine by a Thyroidal Peroxidase1

Recent studies indicate that the deiodination of thyroxine (T4) in vitro by preparations of several tissues may be mediated by a peroxide-peroxidase system. Since the thyroid contains a peroxidase which mediates the oxidation of iodide, the possibility that a similar enzyme exists, which might prove capable of deiodinating T4, was investigated. Studies were performed in which the degradation of 131Ilabeled T4 was assessed in rat thyroids homogenized in Krebs-Ringer phosphate buffer (pH 7.4) containing glucose. Analysis of deiodinating activity was carried out by standard techniques. In the absence of glucose oxidase, a source of hydrogen peroxide, no deiodination occurred in fresh or boiled homogenates. Active deiodination was induced in homogenates, however, by concentrations of glucose oxidase (10 μg/ml) which produced no deiodination in the absence of tissue. The system had many features similar to those of peripheral T4-deiodinating preparations, including: enhancement by moderate dilution, dialysis o...