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Thioredoxin stimulates enzymatic outer ring monodeiodination of reverse triiodothronine
1Department of Medicine, Framingham Union Hospital, Massachusetts 01701.
Endocrinology
|December 1, 1987
Summary
Thioredoxin (Thd) activates a specific enzyme in liver and kidney microsomes to remove reverse triiodothyronine (rT3). This process, crucial for thyroid hormone metabolism, uses NADPH and is distinct from T4 deiodination.
Area of Science:
- Biochemistry
- Endocrinology
- Molecular Biology
Background:
- Thyroid hormone metabolism involves deiodination of thyroxine (T4) and its metabolites.
- Reverse triiodothyronine (rT3) is a non-calorigenic metabolite of T4, and its disposal pathway is of significant physiological interest.
- Microsomal enzymes play a critical role in thyroid hormone biotransformation.
Purpose of the Study:
- To investigate the role of thioredoxin (Thd) and NADPH-thioredoxin reductase in the 5'-monodeiodination of rT3 by rat renal and hepatic microsomes.
- To characterize the kinetic and inhibitory properties of Thd-supported rT3 deiodination compared to dithiothreitol (DTT)-supported reactions.
- To elucidate the potential existence of separate enzymes for rT3 and T4 5'-monodeiodination.
Main Methods:
- Purification of thioredoxin and NADPH-thioredoxin reductase from rat liver cytosol.
- Enzymatic assays measuring 5'-monodeiodination of rT3 and T4 by renal and hepatic microsomes.
- Kinetic analysis (Michaelis constants, maximum velocity) and inhibitor sensitivity studies (propylthiouracil, iopanoate).
- Arrhenius plot analysis to compare activation energies.
Main Results:
- Thioredoxin (Thd), in the presence of NADPH, significantly stimulated the 5'-monodeiodination of rT3 at nanomolar concentrations, with an EC50 of approximately 15 microM.
- Thd-supported rT3 deiodination exhibited Michaelis constants identical to DTT-supported reactions, suggesting the involvement of the same enzyme.
- Thd-supported deiodination was differentially sensitive to propylthiouracil and iopanoate compared to DTT-supported reactions, and activation energies differed from T4 deiodination.
Conclusions:
- Rat renal and hepatic microsomes possess distinct low Km rT3-specific and T4-specific 5'-monodeiodinases.
- The rT3-specific 5'-monodeiodinase can utilize the reducing power of NADPH via the thioredoxin system.
- This Thd-mediated pathway offers a mechanism for rT3 disposal independent of T4 to T3 conversion, highlighting a novel aspect of thyroid hormone regulation.