Toyoda N, Kaptein E, Berry M J, Harney J W, Larsen P R, Visser T J
Thyroid Division, Brigham and Women's Hospital, Boston, Massachusets, USA.
Endocrinology. 1997 Jan;138(1):213-9. doi: 10.1210/endo.138.1.4900.
The bioactivity of thyroid hormone is determined to a large extent by the monodeiodination of the prohormone T4 by the hepatic selenoenzyme type I iodothyronine deiodinase (IDI), i.e. by outer ring deiodination (ORD) to the active hormone T3' or by inner ring deiodination (IRD) to the inactive metabolite rT3. IDI also catalyzes the IRD of T3 and the ORD of rT3' both to T2, as well as the deiodination of different iodothyronine sulfates, e.g. IRD of T3S and ORD of T2S. Previous studies have indicated important differences in catalytic specificity between dog IDI (dID1) and human ID1 (hID1), in particular with respect to the ORD of rT3. This study was done to investigate the relationship between structure and catalytic function of this enzyme by comparing the deiodination of T4, T3, rT3, T3S, and T2S by native dID1 and hID1 in liver microsomes as well as by recombinant wild-type, chimeric and mutated d/hID1 enzymes expressed in HEK293 cells. With both native and recombinant wild-type enzymes, the substrate specificity was T3S > T2S approximately rT3 approximately T4 > T3 for dID1, and rT3 > > T2S approximately T3S > T4 approximately T3 for hID1. Whereas ORD of T4 and of T4, T3, and T3S showed relatively little variation between the different d/hID1 constructs, large differences were found for the ORD of rT3 and T2S. Both reactions were favored by the presence of the amino acids G, E and, in particular, F, present in hID1 at positions 45, 46, and 65, instead of the dID1 residues N, G, and L, respectively. However, although ORD of rT3 was not affected by the presence (hID1) or absence (dID1) of the TGMTR(48-52) sequence, the ORD of T2S was markedly inhibited by the presence of this sequence. Therefore, we have identified structural elements in ID1 that have substrate-specific impacts on deiodination. Our results suggest the specific interaction of the mono-substituted inner ring of the substrates rT3 and T2S but not the disubstituted inner ring of T3, T3S, or T4, with the aromatic ring of F65 in Id1, perhaps by pi-pi interactions.
甲状腺激素的生物活性在很大程度上取决于肝脏硒酶 I 型碘甲状腺原氨酸脱碘酶（IDI）对激素原 T4 的单脱碘作用，即通过外环脱碘（ORD）生成活性激素 T3'，或通过内环脱碘（IRD）生成无活性代谢物 rT3。IDI 还催化 T3 的 IRD 和 rT3' 的 ORD 生成 T2，以及不同碘甲状腺原氨酸硫酸盐的脱碘作用，例如 T3S 的 IRD 和 T2S 的 ORD。先前的研究表明，犬 IDI（dID1）和人 ID1（hID1）在催化特异性上存在重要差异，特别是在 rT3 的 ORD 方面。本研究旨在通过比较肝脏微粒体中天然 dID1 和 hID1 以及在 HEK293 细胞中表达的重组野生型、嵌合和突变 d/hID对 T4、T3、rT3、T3S 和 T2S 的脱碘作用，来研究该酶的结构与催化功能之间的关系。对于天然和重组野生型酶，dID1 的底物特异性为 T3S > T2S ≈ rT3 ≈ T4 > T3，而 hID1 的底物特异性为 rT3 >> T2S ≈ T3S > T4 ≈ T3。虽然 T4、T3 和 T3S 的 ORD 在不同的 d/hID构建体之间变化相对较小，但 rT3 和 T2S 的 ORD 存在很大差异。hID1 在 45、46 和 65 位存在的氨基酸 G、E 尤其是 F，而不是 dID1 相应位置的 N、G 和 L，有利于这两种反应。然而，尽管 rT3 的 ORD 不受 TGMTR(48 - 52)序列存在（hID1）或不存在（dID1）的影响，但 T2S 的 ORD 会受到该序列存在的显著抑制。因此，我们确定了 ID1 中对脱碘作用有底物特异性影响的结构元件。我们的结果表明，底物 rT3 和 T2S 的单取代内环而非 T3、T3S 或 T4 的双取代内环，可能通过 π-π 相互作用与 Id1 中 F65 的芳香环发生特异性相互作用。
