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 (Vmox) 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 Km with respect to T4 remained unchanged (3.2 vs. 2.8 μM T4). Th...
Published Version
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