Thioredoxin Stimulates Enzymatic Outer Ring Monodeiodination of Reverse Triiodothronine*

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Abstract
Thioredoxon (THd) and NADPH-Thd reductase, purified to near homogeneity from rat liver cytosol, stimulated, in the presence of NADPH, the 5''-monodeiodination of rT3 by renal and hepatic microsomes at nanomolar, but not micromolar, substrate concentrations. T4 was not deiodinated at either concentration. Reduced Thd was effective at physiological concentrations in stimulating microsomal rT3 deiodination (EC50 .apprx. 15 .mu.M); Thd-supported microsomal deiodination showed a maximum velocity approximately one third that in the presence of dithiothreitol (DTT), and Thd-supported deiodination, compared to that with DTT, was 10- and 2000-fold more sensitive to inhibition by propylthiouracil and iopanoate, respectively, than was the DTT-supported reaction. The Michaelis constants for rT3 (2.5 nM) were identical for the Thd- and DTT-activated reactions, suggesting that these thiols stimulated deiodination by the same enzyme. Arrhenius plots also revealed comparable activation energies for Thd- and DTT-mediated low Km rT3 monodeiodination; these activation energies were, moreover, distinct from those observed with the low Km T4 deiodination in the presence of DTT. The data suggest that renal and hepatic microsomes contain separate low Km rT3-specific and T4-specific 5''-monodeiodinases and that the rT3-specific monodeiodinase can use the reducing potential of NADPH, via the Thd system. Such an enzyme could mediate the disposal of rT3, the noncalorigenic metabolite of T4, independently of the conversion of T4 to T3.

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