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Abstract: The three distinct but related isotypes of the iodothyronine deiodinase family: D1, D2, and D3, have been amply studied in vertebrate homeotherms and to a lesser extent in ectotherms, particularly in reptiles. Here, we report the molecular and kinetic characteristics of both the native and the recombinant hepatic D3 from the pine snake Pituophis deppei (PdD3). The complete PdD3 cDNA (1680bp) encodes a protein of 287 amino acids (aa), which is the longest type 3 deiodinase so far cloned. PdD3 shares 78% identity with chicken and 71% with its other orthologs. Interestingly, the hinge domain in D3s, including PdD3, is rich in proline. This structural feature is shared with D1s, the other inner-ring deiodinases, and deserves further study. The kinetic characteristics of both native and recombinant PdD3 were similar to those reported for D3 in other vertebrates. True K m values for T3 IRD were 9 and 11nM for native and recombinant PdD3, respectively. Both exhibited a requirement for a high concentration of cofactor (40mM DTT), insensitivity to inhibition by PTU (>2mM), and bisubstrate, sequential-type reaction kinetics. In summary, the present data demonstrate that the liver of the adult pine snake P. deppei expresses D3. Furthermore, this is the first report of the cloning and expression of a reptilian D3 cDNA. The finding of hepatic D3 expression in the adult pine snake P. deppei is consistent with results in adult piscine species in which the dietary T3 content seems to regulate liver deiodinase expression. Thus, our present results support the proposal that hepatic D3 in adult vertebrates plays a sentinel role in avoiding an inappropriate overload of exogenous T3 secondary to feeding in those species that devour the whole prey. [ABSTRACT FROM AUTHOR]

Substrate availability has been thought to be a major regulator of the outer-ring deiodinating pathway (ORD) in fish. However, current information strongly suggests that while fish iodothyronine deiodinase type 2 (D2) responds to iodothyronines in the same manner as its mammalian counterpart, fish deiodinase type 1 (D1) exhibits a distinct response. Furthermore, 3,5-T2, generally considered to be an inactive product of iodothyronine metabolism, has recently been described as bioactive, but its effects upon D1 and D2 are not yet known. We examined the effect that short-term immersion in T4, T3, and 3,5-T2 (0.1 μM; 12 or 24 h) exerts on both D1 and D2 activities and on the levels of expression of D1 and D2 mRNAs in killifish liver. In agreement with previous reports in teleosts, no iodothyronine exerted a significant effect on D1 enzymatic activity. However, all three iodothyronines significantly decreased D2 activity. Furthermore, at 24 h post-immersion T4, T3, and 3,5-T2 inhibited both D1 and D2 transcription. Together, the present results confirm the differential effect of iodothyronines upon the hepatic ORD pathway in fish and show that this effect can occur at a transcriptional level. Furthermore, we provide the first evidence that 3,5-T2 can affect both activity and transcription of hepatic deiodinases in teleosts. [Copyright &y& Elsevier]

The stereospecific removal of iodine from thyroid hormones is an essential first step for T3 action and is catalyzed by three different deiodinases: D2 and D3 remove iodine only from the outer or inner ring, respectively, whereas D1 catalyzes both pathways. We used in silico predictions from vertebrate deiodinase sequences to identify two domains: the N-terminal variable region (VR) containing the transmembrane, hinge and linker domains, and the conserved or globular region (CR). Given the high sequence and structural identity of the CR among paralogs as well as of the VR among orthologs but not paralogs, we hypothesized that both the catalytic properties and the subcellular localization rely on the VR. We used shark D2 and D3 as templates to build the chimeric enzymes D2VR/D3CR and D3VR/D2CR. Biochemical characterization revealed that D3VR/D2CR has inner-ring deiodination activity and T3 as preferred substrate, whereas D2VR/D3CR showed no deiodinating activity. Also, D2VR/D3CR and D3VR/D2CR reside in the endoplasmic reticulum and plasmatic membrane, respectively, as do their D2 and D3 wild-type counterparts. We conclude that the VR determines the subcellular localization and is critical in defining the catalytic properties and activity of thyroid hormone deiodinases. [ABSTRACT FROM AUTHOR]