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.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism