Iodothyronine-Induced Catalatic Activity of Thyroid Peroxidase

Abstract

Iodothyronines induced catalatic (H₂0₂-decomposing) activity of thyroid peroxidase and lactoperoxidase, the effect increasing in the order of thyroxine (T₄) > triiodothyronine (T₃) > diiodothyronine (T₂). The iodothyronines served as electron donors in the peroxidase reactions, and during the reactions the catalytic intermediate of thyroid peroxidase was confirmed to be Compound II for T₄ and Compound I for T₃ and T₂ and from the Soret absorption spectra obtained by stopped-flow measurements. Rate constants for the reac-tions between T₄ and Compound II, T₃ and Compound I, and T₂ and Compound I were estimated at 1.9×10⁵, 1.3×10⁶, and 7.1×10⁵ M^-1•s^-1, respectively. Unlike the case of thyroid peroxidase, the catalytic intermediate of lactoperoxidase observed during the oxidation of iodothyronines was invariably Compound II. From these and other data it was concluded that thyroid peroxidase catalyzed one-electron oxidation of T₄ and two-electron oxidations of T₂ and T₃ while lactoperoxidase catalyzed exclusively one-electron oxidation of the iodothyronines. Iodide was released during the enzymatic oxidation of iodothyronines, irrespective of the mechanism of one-electron and two-electron oxidations. The amount of released iodide increased in the order of T₄ > T₃ >T₂. The iodothyronines-induced catalatic activity of these peroxidases was ascribable to the release of iodide, but it was also found that the iodide-enhanced catalatic activity was stimulated by iodothyronines. In this case the effect of iodothyronines was greater in the order of T₂ > T₃> T₄ which was consistent with the order of iodothyronine activation for the iodinium cation transfer from enzyme to acceptor.