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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.
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