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Preincubation of Thyroxine with Sulfhydryl-Reducing Agents Does Not Stimulate Thyroxine Inner or Outer Ring Deiodination^*

Division of Endocrinology, Department of Medicine, University of North Carolina School of Medicine Chapel Hill, North Carolina 27514;
The Division of Endocrinology and Metabolism, Department Medicine, University of Massachusetts Medical School , Worcester, Massachusetts 01605

Address requests for reprints to: Robert D. Utiger, M.D., Department of Medicine, University of North Carolina School of Medicine, Chapel Hill, North Carolina 27514.

Abstract

Sulfhydryl reagents stimulate enzymatic conversion of T₄ to T₃ and rT₃. A recent study suggested that such reagents stimulated T₄ 5'-deiodination by a direct interaction with T₄. We therefore tested the ability of dithiothreitol (DTT) and other sulfhydryl reagents to enhance the susceptibility of T₄ and rT₃ to 5'-deiodination by liver homogenates and of T₄ to 5- deiodination by placental homogenates. Preincubation of T₄ with DTT in concentrations ranging from 0.5–80 mM did not result in increased T₃ production from T₄ in rat liver homogenates, nor was T₃ production increased by preincubation of T₄ with reduced glutathione or mercaptoethanol. Preincubation of rT₃ with DTT also did not result in increased rT₃ degradation by liver homogenates. T₄ 5-deiodination to rT₃ by rat and human placental homogenates was not consistently increased by preincubation of T₄ with DTT in concentrations ranging from 2.25– 450 mM. These results do not support the hypothesis that sulfhydryl stimulation of T₄ deiodination occurs as a result of sulfhydryl-T₄ interaction. (Endocrinology 113: 851,1983)

Footnotes

^* Supported by USPHS Grants AM-18919, AM-07302, AM-26112,AM-27850, AM-07129, and RR-46.

Received October 8, 1982.