Selenocysteine confers the biochemical properties characteristic of the type I iodothyronine deiodinase.
The conversion of thyroxine to 3,5,3'-triiodothyronine (T3) is the first step in thyroid hormone action, and the Type I iodothyronine deiodinase supplies most of this extrathyroidal T3 in the rat. We found that the cDNA coding for this enzyme contains an in-frame UGA encoding the rare amino acid selenocysteine. Using site-directed mutagenesis, we have converted selenocysteine to cysteine and expressed the wild-type and cysteine mutant enzymes in JEG-3 cells by transient transfection. The kinetic properties of the transiently expressed wild-type enzyme are nearly identical to those reported for rat liver Type I deiodinase. Substitution of sulfur for selenium causes a 10-fold increase in the Km of the enzyme for the favored substrate 3,3',5'-triiodothyronine (rT3), a 100-fold decrease in the sensitivity of rT3 deiodination to competitive inhibition by gold and a 300-fold increase in the apparent Ki for uncompetitive inhibition by 6-n-propylthiouracil. These results demonstrate that selenium is responsible for the biochemical properties which characterize Type I iodothyronine monodeiodination.
Berry
M J
MJ
Howard Hughes Medical Institute Laboratory, Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts.
Kieffer
J D
JD
Harney
J W
JW
Larsen
P R
PR
D000818:Animals; D003545:Cysteine / Q000031:analogs & derivatives* / Q000378:metabolism; D004247:DNA / Q000235:genetics; D007453:Iodide Peroxidase / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D016297:Mutagenesis, Site-Directed; D010957:Plasmids; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D011919:Rats, Inbred Strains; D012643:Selenium / Q000378:metabolism*; D017279:Selenocysteine; D014162:Transfection; D014284:Triiodothyronine / Q000378:metabolism; D014981:Xenopus
D012333:RNA, Messenger; D014284:Triiodothyronine; D017279:Selenocysteine; D004247:DNA; D007453:Iodide Peroxidase; D012643:Selenium; D003545:Cysteine