Inhibitory effect of unlabeled iodothyronines on the deiodination of labeled thyroid hormones by cultured hepatocarcinoma cells.

Inhibitory effects of unlabeled iodothyronines on the metabolism of thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (reverse T3, rT3) were investigated in continuously cultured monkey hepatocarcinoma cells which showed a rapid metabolism of the thyroid hormones. Nonphenolic ring deiodination of [3',5'-125I]-T4 and [3'-125I]-T3 was strongly inhibited by excess T3, 3,5-diiodothyronine (3,5-T2) and T4, whereas rT3 was the least effective inhibitor. Phenolic ring deiodination of [3,5'-125I]-rT3 was strongly affected by excess unlabeled rT3. However, the inhibitory effect of T4, T3 and 3,5-T3 was much weaker than that of rT3. It was concluded that rT3 is apparently the most effective inhibitor of phenolic ring deiodination but the least effective inhibitor of nonphenolic ring deiodination.