In all classes of vertebrates, the deiodination of the prohormone T(4) to T(3) represents an essential activation step in thyroid hormone action. The possible presence of @entity9117 deiodinase activity in protochordates has been demonstrated in vivo. Recent molecular cloning of the genomes and transcripts of several ascidian species allows further investigation into thyroid-related processes in ascidians. A cDNA clone from @entity25598 (hrDx) was found to have significant homology (30% @entity535 identity) with the @entity9117 deiodinase gene sequences from vertebrates, including the presence of an in-frame UGA codon that might encode a @entity16266 ( @entity16267 ) in the active site. Because it was not certain that the 3' untranslated region (UTR) contained a @entity16267 insertion sequence (SECIS) element essential for @entity16267 incorporation, a chimeric expression vector of the hrDx coding sequence and the @entity35 deiodinase SECIS element was produced, as well as an expression vector containing the intact hrDx cDNA. COS, @entity435 , and HEK cells were transfected with these vectors, and deiodinase activity was measured in cell homogenates. Outer-ring deiodinase activity was detected using both T(4) and reverse T(3) as substrates, and activity was enhanced by the presence of the reductive cofactor @entity3902 . The enzyme activity was optimal during incubation between 20 and 30 C (pH 6-7) and was strongly inhibited by @entity13394 . The @entity25598 deiodinase appears to be a high Michaelis-Menten constant (K(m)) enzyme (K(m) reverse T(3), 2 microM; and K(m) T(4), 4 microM). Deiodinase activity was completely lost upon the substitution of the @entity16267 residue in the putative catalytic center by either @entity462 or @entity493 . Transfection of the full-length hrDx cDNA produced deiodinase activity confirming the presence of a SECIS element in the 3'UTR, as revealed by the SECISearch program. In conclusion, our results show, for the first time, the existence of an ascidian @entity9117 outer-ring deiodinase. This raises the hypothesis that, in protochordates, the prohormone T(4) is activated by enzymatic outer-ring deiodination to T(3).

An ascidian homolog of vertebrate XXXX deiodinases.

@entity9117 :: ('CHEBI:24864', 'Chemical') :: ['iodothyronine'] @entity462 :: ('MESH:D003545', 'Chemical') :: ['cysteine'] @entity25598 :: ('7729', 'Species') :: ['Halocynthia', 'Halocynthia roretzi'] @entity535 :: ('CHEBI:33704', 'Chemical') :: ['amino acid'] @entity16267 :: ('CHEBI:30003', 'Chemical') :: ['SeC'] @entity3902 :: ('MESH:D004229', 'Chemical') :: ['dithiothreitol'] @entity16266 :: ('MESH:D017279', 'Chemical') :: ['selenocysteine'] @entity13394 :: ('MESH:D006051', 'Chemical') :: ['gold-thioglucose'] @entity493 :: ('CHEBI:16449', 'Chemical') :: ['alanine'] @entity35 :: ('10116', 'Species') :: ['rat'] @entity435 :: ('CHEBI:42485', 'Chemical') :: ['CHO']
@entity9117 :: (CHEBI:24864,Chemical) :: ['iodothyronine']
