Iodothyronine-induced catalatic activity of thyroid peroxidase

Iodothyronines induced catalatic (H2O2-decomposing) activity of thyroid peroxidase and lactoperoxidase, the effect increasing in the order of thyroxine (T4) greater than triiodothyronine (T3) greater than 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 reactions between T4 and Compound II, T3 and Compound I, and T2 and Compound I were estimated at 1.9 x 10(5), 1.3 x 10(6), and 7.1 x 10(5) 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 greater than T3 greater than 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 greater than T3 greater than T4, which was consistent with the order of iodothyronine activation for the iodinium cation transfer from enzyme to acceptor.