Effect of propylthiouracil on 125I-L-triiodothyronine binding to the nuclei and on malic enzyme activity in rat liver cytosol.

The effect of propylthiouracil (PTU), triiodothyronine (T3) and thyroxine (T4) on malic enzyme activity of rat liver cytosol and on the binding of 125I-L-triiodothyronine to nuclear fraction was examined. Significant decrease of in vitro binding of 125I-T3 to the liver nuclei was found in rats fed PTU for 12 h, 24 h and 120 h. Hepatic malic enzyme activity was unchanged after 12 h, but markedly decreased after 24 h and 120 h of PTU treatment. In 120 h PTU fed animals the effect of T3 on malic enzyme activity was five times higher in comparison to an equimolar dose of T4. Finally, the effect of T4 on malic enzyme activity was observed which might result from the intrinsic activity T4 under the conditions of decreased T4 to T3 conversion due to PTU feeding. The obtained results show that there is a certain correlation between nuclear T3 binding and cytosol malic enzyme activity which is presumably induced directly through m-RNA synthesis stimulated by thyroid hormones.