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  • Title: Evidence that cysteine, not selenocysteine, is in the catalytic site of type II iodothyronine deiodinase.
    Author: Berry MJ, Kieffer JD, Larsen PR.
    Journal: Endocrinology; 1991 Jul; 129(1):550-2. PubMed ID: 1829034.
    Abstract:
    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.
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