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J Biol Chem, Vol. 273, Issue 39, 25230-25236, September 25, 1998
From the Departments of Medicine and Physiology, Dartmouth Medical
School, Lebanon, New Hampshire 03756
The iodothyronine deiodinases are a family of
oxidoreductases that catalyze the removal of iodide from thyroid
hormones. Each of the three isoforms contain selenocysteine at its
active site and several cysteine residues that may be important for
catalytic activity. Of particular interest in the type I deiodinase
(D1) is Cys124, which is vicinal to the
selenocysteine at position 126, and Cys194, which has been
conserved in all deiodinases identified to date. In the present
studies, we have characterized the functional properties of C124A,
C194A, and C124A/C194A D1 mutants, which were prepared by site-directed
mutagenesis and expressed in COS-7 cells. In broken cell preparations,
the sensitivity of the mutants to the selective D1 inhibitors
propylthiouracil and aurothioglucose were unaltered. Mutagenesis at the
Cys124 position was associated with a 7-11-fold increase
in the Km of dithiothreitol, whereas
Vmax values remained largely unchanged. However, both mutations resulted in marked decreases in
Vmax values when glutathione or a reconstituted
thioredoxin cofactor system were used in the assay. In contrast to the
results of these in vitro studies, no impairment in
deiodinating capability was noted in intact cells expressing equivalent
levels of the mutant constructs. These studies demonstrate that
Cys124 and Cys194 influence the reactivity of
the D1 with thiol cofactors in in vitro assay systems but
are not determinants of the sensitivity of the enzyme to
propylthiouracil and aurothioglucose. Furthermore, the observation that
the cysteine mutants are fully active in intact cells demonstrates that
the results of commonly used broken cell assays do not accurately
predict the activity of the D1 in intact cells and suggests that
glutathione and thioredoxin are not the major thiols utilized in
vivo to support D1 activity.
Conserved Cysteines in the Type 1 Deiodinase Selenoprotein Are
Not Essential for Catalytic Activity
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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