Phenolic and Nonphenolic Ring Iodothyronine Deiodinases from Rat Thyroid Gland

Publisher Website
Google Scholar
Abstract
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 unlabeled T4, in each case inhibited the formation of T3 by 30% and 70% when the alcohol was 1.25% and 5% (vol/vol), respectively. The enzyme preparation briskly catalyzed 5′-deiodination of added rT3 to T2, the activity decreasing between pH 6.5–8.0. Only meager conversion of T3 to T2 occurred. Although very little rT3, determined by RIA, accumulated in reaction mixtures when T4 was incubated with the thyroid preparation, large amounts of T2, determined by both RIA and chromatography, were formed. These results indicate 5-deiodination of T4, followed by 5′-deiodination of rT3 to T2. The conversion of T4 to T2 in this pathway increased between H 6.5–8.0 despite the more rapid deiodination of rT3 to T2 at the lower pH. Thus, iodothyronine monodeiodinations in the thyroid particulate fraction are similar to those already described in similar preparations of liver and kidney. T3 produced in the thyroid by deiodination of T4 might play a role in regulating the metabolic state of the gland. Although the importance of such T3 for the overall production of the hormone in the rat remains to be evaluated, it very likely contributes, along with hormone released from thyroglobulin, to overall thyroid secretion.