Induction of the thyroxine (T4) to triiodothyronine (T3) converting enzyme in rat liver by thyroid hormones and analogs.

In thyroidectomized, unsubstituted rats the T4 to T3 converting activity of liver homogenate is reduced to about 30% of that in unoperated control animals. The enzyme activity can be reinduced dose-dependently with T4. To achieve a normal activity, high, non-physiological plasma T4 concentrations are needed. Plasma T3 levels are much better correlated to the T4 to T3 converting activity. Pure T3 proved to be a more potent enzyme inducer than T4. No difference could be detected between L- and D-T3. Tyrosine, diiodotyrosine, 3,3'-diiodothyronine and 3,3',5'-T3 (reverse-T3) showed no inductive effect for the enzyme. These results demonstrate that the T4 to T3 converting enzyme is specific and shows regulatory properties.