Phenolic and Nonphenolic Ring Iodothyronine Deiodinases from Rat Thyroid Gland

A particulate fraction of rat thyroid gland sedimenting between 1,500–100,000 ×g contains enzymatic activities that catalyze the sequential deiodination of T4 to 3,3′-diiodothyronine (T2) via both T3 and rT3. The T4-5′-deiodinase of this particulate fraction has been studied in some detail using a specific T3 RIA. The presence of a sulfhydryl compound such as dithiothreitol was required for activity which was optimal at pH 6.5 and was twice as high for fractions prepared at 7.4 and assayed at 6.5 than for material both prepared and assayed at 6.5. The apparent Km for T4 was 3.1 μM, and the Vmax was 53 pmol/mg protein·min. Activity was destroyed by heating for 30 min at 60 C, was completely inhibited by 50 μM 6-propyl-2- thiouracil, but was unaffected by methimazole. T3 production values agreed closely when measured by RIA and by paper chromatography using [125I]T4 as substrate. Propanol in reaction mixtures, contributed by labeled T4 stock solutions (50% propanol-50% H2O) or added to mixtures containing u...