Evidence that cysteine, not selenocysteine, is in the catalytic site of type II iodothyronine deiodinase.
Recent cloning of the cDNA for Type I iodothyronine deiodinase revealed that the mRNA contains a UGA codon encoding the amino acid selenocysteine. Mutagenesis of the selenocysteine codon to a cysteine codon produced a protein with lower deiodinase activity. The presence or absence of selenocysteine in Type II deiodinase, which differs from the Type I enzyme in a number of parameters, has not been determined. Gold inhibits the activity of both the Type I deiodinase and the only other known eukaryotic selenocysteine-enzyme, glutathione peroxidase. Substitution of cysteine for selenocysteine in Type I deiodinase reduced its sensitivity to inhibition by gold 500-fold. We found that gold thioglucose was a competitive inhibitor with respect to the iodothyronine substrate of both deiodinases. However, the Type II enzyme from brown fat and pituitary was 100 to 1000-fold less sensitive to gold than was Type I activity in liver and pituitary, similar to the results with the cysteine-substituted Type I enzyme. This suggests that Type II deiodinase contains cysteine instead of selenocysteine in the active site.
Berry
M J
MJ
Department of Medicine, Brigham and Women's Hospital, Boston.
Kieffer
J D
JD
Larsen
P R
PR
D002001:Adipose Tissue, Brown / Q000201:enzymology; D000818:Animals; D006051:Aurothioglucose / Q000494:pharmacology; D001665:Binding Sites; D001667:Binding, Competitive; D003545:Cysteine / Q000031:analogs & derivatives* / Q000378:metabolism*; D005979:Glutathione Peroxidase / Q000037:antagonists & inhibitors; D007453:Iodide Peroxidase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008099:Liver / Q000201:enzymology; D008297:Male; D010902:Pituitary Gland / Q000201:enzymology; D051381:Rats; D011919:Rats, Inbred Strains; D012643:Selenium / Q000378:metabolism*; D017279:Selenocysteine
D017279:Selenocysteine; D006051:Aurothioglucose; D007453:Iodide Peroxidase; D005979:Glutathione Peroxidase; D012643:Selenium; D003545:Cysteine