Thyroxine-5&-Deiodinase of Rat Thyroid, But Not that of Liver, Is Dependent on Thyrotropin*

Abstract

T₄^-5′-deiodinase activity was assayed in particulate fractions (sedimenting between 1,500–100,000 × g) of thyroids and livers of normal rats, rats made hypothyroid by surgical hypophysectomy, hypophysectomized rats maintained with daily T₄ replacement, sham-hypophysectomized rats, and rats made hypothyroid and goitrous by chronic ingestion of methimazole (MMI). Rates of conversion of T₄ to T₃ catalyzed by the fractions under standard conditions in the presence of 5 mM dithiothreitol were measured to assess the effects of the various treatments (1-month duration) on enzyme activity. T₃ was measured by a specific RIA in ethanol extracts of the reaction mixtures. Serum levels of TSH and T₄ indicated efficacy of the treatments. The maximal velocity (V_mox) for the hepatic enzyme of the goitrous rats, calculated by the method of Lineweaver and Burk, was, as expected, only one third of normal (15.5 vs. 52 pmol T₃-min/mg protein), while the apparent K_m with respect to T₄ remained unchanged (3.2 vs. 2.8 μM T₄). The V_max for the sham-hypophysectomized animals, essentially identical to that for normal controls, was reduced to one fifth of its magnitude by hypophysectomy (10.8 vs. 57 pmol T₃-min/mg protein), but was maintained at control values by daily T₄ replacement (1.7 μg/100 g BW) begun on the day of surgical intervention, indicating dependence of the hepatic enzyme on thyroid hormone and not on a hypophyseal factor such as GH, whose synthesis and release is thyroid hormone dependent. Apparent K_m values for the hepatic enzyme of various groups of hypophysectomized rats were all normal. On the other hand, enzyme activity was augmented in the thyroid preparations from rats treated with MMI; the V_max increased from the normal value of 212 to 408 pmol T₃ min/mg protein, with the apparent K_m remaining essentially constant (4.6 μM T₄ for normal vs. 4.0 μ.M T₄ for goitrous rats). In the glands from MMI-treated rats, potential T₄-5′&-deiodinase activity, expressed per μg DNA (when T₄ was saturating), was 3 times normal, indicating a 3-fold increase in the enzyme activity per cell. Activity in the preparations from thyroids of hypophysectomized rats, examined either 10 or 30 days posthypophysectomy, was too low to be measured; less than 0.25 pmol T₃-min/mg protein were formed when the T₄ concentration was 1.85 μM. The results of these experiments demonstrate that the activity of thyroid T₄-5′-deiodinase, unlike that of the liver enzyme, is not related to thyroid hormone levels, but is determined by the degree of stimulation of the gland by TSH. In addition, the absence of detectable deiodinase activity in the thyroids of rats 10 days after hypophysectomy indicates that the enzyme has a half-life of the order of 1 or 2 days.