A Study of the Properties of the Enzyme in Rat Liver that Deiodinates 3,3′,5′-Triiodothyronine to 3,3′-Diiodothyronine*

Abstract

Studies were performed to assess the factors that influence the metabolism of [¹²⁵I]rT₃ in homogenates of rat liver. [¹²⁵I]rT₃ was metabolized with great rapidity; in 10% homogenates, rT₃ was completely degraded within 30 min, giving rise to nearly equal proportions of two ¹²⁵I-labeled metabolites, [¹²⁵I]3,3′-diiodothyronine ([¹²⁵I]3,3′-T₂) and [¹²⁵I]iodide. This process appeared to be enzyme mediated, since it was heat labile, temperature dependent, concentration dependent, and saturable with stable rT₃. Aerobic preincubation markedly impaired the reaction, while anaerobic preincubation did not. These effects appeared to be due, respectively, to depletion and preservation of tissue glutathione (GSH) content during preincubation. Thus, the loss of activity seen after aerobic preincubation was prevented by the addition of either GSH itself or a NADPHgenerating system. GSH (5 HIM) stimulated [¹²⁵I]rT₃ degradation in dilute homogenates (1–5%) but not in concentrated homogenates (10–20%). Further evidence suggesting the importance of sulfhydryl groups in this reaction was obtained in studies with the mercaptan, dithiothreitol, which stimulated the reaction, and the sulfhydryl oxidant, diamide, which produced marked inhibition. Diiodotyrosine was weakly inhibitory when added to homogenates in vitro, but was markedly inhibitory to in vitro hepatic degradation of rT₃ when administered in vivo. Its effect appeared to be unrelated to the release of inorganic iodine during its metabolism, since iodide was without effect even at high concentrations. Rather, the effect of diiodotyrosine may have been related to the utilization of the NADPH, a cofactor for iodotyrosine deiodinase. Metabolism of [¹²⁵I]rT₃ was retarded in hypothyroidism and accelerated in hyperthyroidism. The process was markedly impaired after a 48-h fast despite the administration of replacement doses of T₄ to prevent hypothyroidism. The in vivo administration of several pharmacological agents produced varying degrees of inhibition; this effect was slight with dexamethasone, marked with iocetamic acid, and almost complete with propylthiouracil and Amiodarone. The effects of all of these altered physiological states and pharmacological agents on rT₃ metabolism were qualitatively similar to those previously shown to occur in the rat liver in vitro with respect to the conversion of T₄ to T₃. These data indicate that the 5′-monodeiodinase that converts T4 to T3 and that which converts rT₃ to 3,3′-T₂ are either the same enzyme or, if different, have remarkably similar properties.