Endocrinology, Vol 110, 241-245, Copyright © 1982 by Endocrine Society

ARTICLES

Effect of sulfhydryl reagents on the conversion of thyroxine to 3,5,3'- triiodothyronine: direct action on thyroxine molecules

Y Ozawa, T Shimizu and Y Shishiba

Sulfhydryl reagents are good stimulators of the enzymatic conversion of T4 to T3. Previous studies have suggested that the underlying mechanism for this property is that these reagents may serve as a cofactor in the reaction, or they maintain the enzyme in its active reduced form. We examined another possibility, namely that sulfhydryl reagents affect T4 itself and hence increase T3 generation by the enzyme. The substrate T4 and sulfhydryl reagents dithiothreitol, mercaptoethanol, and reduced glutathione were incubated at 37 C for 1 h and then combined with rat liver homogenate and incubated for additional 15 min to permit the enzymatic formation of T3. For control, substrate T4 was not preincubated with reagents, and those reagents were added just before the incubation. Preincubation of T4 with these reagents markedly enhanced subsequent T3 generation by the homogenate. Without preincubation, however, reduced glutathione and mercaptoethanol did not enhance T3 generation by simultaneous incubation with the homogenate. Dithiothreitol increased T3 generation when introduced into the incubation medium simultaneously with the homogenate and incubated only 15 min. However, the magnitude of stimulation was far less than that obtained from T4 preincubated with this reagent. These procedures did not alter the nonenzymatic generation of T3 significantly. These results suggest that sulfhydryl reagents can affect T4 itself and make it more susceptible to enzymatic outer ring monodeiodination.