D. Salvatore et al., TYPE-3 IODOTHYRONINE DEIODINASE - CLONING, IN-VITRO EXPRESSION, AND FUNCTIONAL-ANALYSIS OF THE PLACENTAL SELENOENZYME, The Journal of clinical investigation, 96(5), 1995, pp. 2421-2430
Type 3 iodothyronine deiodinase (D3) catalyzes the conversion of T-4 a
nd T-3 to inactive metabolites. It is highly expressed in placenta and
thus can regulate circulating fetal thyroid hormone concentrations th
roughout gestation. We have cloned and expressed a 2.1-kb human placen
tal D3 cDNA which encodes a 32-kD protein with a K-m of 1.2 nM for 5 d
eiodination of T-3 and 340 nM for 5' deiodination of reverse T-3. The
reaction requires DTT and is not inhibited by 6n-propylthiouracil. We
quantitated transiently expressed D3 by specifically labeling the prot
ein with bromoacetyl [I-125]]T-3. The K-cat/K-m ratio for 5 deiodinati
on of T-3 was over 1,000-fold that for 5' deiodination of reverse T-3.
Human D3 is a selenoenzyme as evidenced by (a) the presence of an in
frame UGA codon at position 144, (b) the synthesis of a 32-kD Se-75-la
beled protein in D3 cDNA transfected cells, and (c) the presence of a
selenocysteine insertion sequence element in the 3' untranslated regio
n of the mRNA which is required for its expression. The D3 selenocyste
ine insertion sequence element is more potent than that in the type 1
deiodinase or glutathione peroxidase gene, suggesting a high priority
for selenocysteine incorporation into this enzyme. The conservation of
this enzyme from Xenopus laevis tadpoles to humans implies an essenti
al role for regulation of thyroid hormone inactivation during embryolo
gical development.