In frogs such as Rana and Xenopus metamorphosis does not occur in the absence of a functional thyroid gland. Previous studies indicated that coordinated development in frogs requires tissue and stage-dependent type II (D2) and type III (D3) iodothyronine deiodinase expression patterns, to obtain requisite levels of intracellular T3 in tissues at the appropiate stages of metamorphosis. No type I iodothyronine deiodinase (D1), defined as T4 or rT3 outer ring deiodinase (ORD) activity with Michaelis constant (K_m) values in the micromolar range and sensitivity to 6-propyl-2-thiouracil (6-PTU), could be detected in tadpoles sofar.

We have obtained a X. laevis D1 cDNA clone from brain tissue. The complete sequence of this clone (1.1 kb, including poly A tail) encodes an ORF of 252 amino acid residues with high homology to other vertebrate D1 enzymes. The core catalytic center includes a UGA-encoded selenocysteine (SeC) residue, and the 3' UTR (about 300 nt) contains a SECIS (selenocysteine insertion sequence) element. Transfection of cells with an expression vector containing the full-length cDNA resulted in generation of significant deiodinase activity in the homogenates. The enzyme displayed ORD activity with T4 (K_m 0.5 µM) and rT3 (K_m 0.5 µM) and inner ring deiodinase activity with T4 (K_m 0.4 µM). Recombinant Xenopus D1 was essentially insensitive to inhibition by 6-PTU (IC₅₀ > 1 mM), but was sensitive to gold thioglucose (IC₅₀ 0.1 µM) and iodoacetate (IC₅₀ 10 µM). Because the residue 2 positions downstream from the SeC is Pro in Xenopus D1 but Ser in all cloned PTU-sensitive D1 enzymes, we prepared the Pro132Ser mutant of Xenopus D1. The mutant enzyme showed strongly increased ORD activity with T4 and rT3 (K_m about 4 µM) and was highly sensitive to 6-PTU (IC₅₀ 2 µM). Little native D1 activity could be detected in Xenopus liver, kidney brain and gut, but significant D1 mRNA expression was observed in juvenile brain and in adult liver and kidney.

These results indicate the existence of a 6-PTU-insensitive D1 enzyme in Xenopus laevis tissues, but its role during tadpole metamorphosis remains to be defined.