STRUCTURE-ACTIVITY-RELATIONSHIPS FOR THYROID-HORMONE DEIODINATION BY MAMMALIAN TYPE-I IODOTHYRONINE DEIODINASES

The bioactivity of thyroid hormone is determined to a large extent by the monodeiodination of the prohormone T-4 by the hepatic selenoenzyme type I iodothyronine deiodinase (ID1), i.e. by outer ring deiodinatio n (ORD) to the active hormone T-3, or by inner ring deiodination (IRD) to the inactive metabolite rT(3). ID1 also catalyzes the IRD of T-3 a nd the ORD of rT(3), both to T-2, as well as the deiodination of diffe rent iodothyronine sulfates, e.g. IRD of T3S and ORD of T2S. Previous studies have indicated important differences in catalytic specificity between dog ID1 (dID1) and human ID1 (hID1), in particular with respec t to the ORD of rT(3). This study was done to in investigate the relat ionship between structure and catalytic function of this enzyme by com paring the deiodination of T-4, T-3, rT(3), T3S, and T2S by native dID 1 and hID1 in liver microsomes as well as by recombinant wild-type, ch imeric and mutated d/hID1 enzymes expressed in HEK293 cells. With both native and recombinant wild-type enzymes, the substrate specificity w as T3S > T2S approximate to rT(3) much greater than T-4 > T-3 for dID1 , and rT(3) greater than or equal to T2S approximate to T3S > T-4 appr oximate to T-3 for hID1. Whereas ORD of T-4 and of T-4, T-3, and T3S s howed relatively little variation between the different d/hID1 constru cts, large differences were found for the ORD of rT(3) and T2S. Both r eactions 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 rT(3) was not affected by the presence (hID1) or absence (dID1) of th e TGMTR(48-52) sequence, the ORD of T2S was markedly inhibited by the presence of this sequence. Therefore, we have identified structural el ements in ID1 that have substrate-specific impacts on deiodination. Ou r results suggest the specific interaction of the mono-substituted inn er ring of the substrates rT(3) and T2S but not the disubstituted inne r ring of T-3, T3S, or T-4 with the aromatic ring of F65 in ID1, perha ps by pi-pi interactions.