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MARLA J. BERRY, J. DAVID KIEFFER, P. REED LARSEN, Evidence that Cysteine, not Selenocysteine, is in the Catalytic Site of Type II Iodothyronine Deiodinase, Endocrinology, Volume 129, Issue 1, 1 July 1991, Pages 550–552, https://doi.org/10.1210/endo-129-1-550
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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 cysteinesubstituted Type I enzyme. This suggests that Type II deiodinase contains cysteine instead of selenocysteine in the active site.
