Inhibitory Effect of Unlabeled Iodothyronines on the Deiodination of Labeled Thyroid Hormones by Cultured Hepatocarcinoma Cells

Abstract

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.