3′-Monoidothyronine degradation in rat liver homogenate: Enzyme characteristics and documentation of deiodination by high-pressure liquid chromatography

Characteristics of 3′-monoiodothyronine (3′-T₁) degradation were examined in vitro in rat tissue homogenates. In rat liver homogenates, 3′-T₁ degradation was optimal at pH 7.4, and was dependent upon time, temperature, and tissue concentration. The Michaeli's constant (Km) = 0.84 μmol/L. 3′-T₁ degradation was enhanced by dithiothreitol and inhibited by propylthiouracil, sodium ipodate, ANS, and sodium azide but not by methimazole. Animals that fasted for three days had signficant reductions in both hepatic T₄ to T₃ conversion (199 ± 12 v 116 ± 12 pg T₃ generated/mg protein; P < 0.001) and 3′-T₁ degradation (588 ± 31 v 148 ± 53 pg 3′-T₁ degraded/mg protein; P < 0.001). To document that 3′-T₁ degradation was occurring by deiodination, both liver and kidney homogenates were incubated with ¹²⁵I-3′-T₁ (∼ 3 μCi; 13.1 nmol/L). The reaction products were separated on a reverse-phase high pressure liquid chromatography (HPLC) column. In both tissues an iodide peak was generated, and no other radiolabeled peaks appeared exceptor for ¹²⁵I-3′-T₁. These data suggest that 3′-T₁ is metabolized by phenolic-ring monodeiodination and is enzymic in nature.