Biochemical characteristics of iodothyronine monodeiodination by rat liver microsomes: the interaction between iodothyronine substrate analogs and the ligand binding site of the iodothyronine deiodinase resembles that of the TBPA-iodothyronine ligand binding.

T4-analogs modified at the aliphatic side chain R are substrates for rat liver microsomal iodothyronine-5'(3')-deiodinase (ITH-D). The variation of the substrate constitution allows a mapping of the ITH-D substrate ligand-binding site. Highest affinity for the ITH-5'(3')-D was presented by Tetrac among a homologous series of carboxylic acid side chain analogs. A net negative charge of the side chain and/or the blockage of the amino function increase the affinity of the enzyme-ligand interaction. However, not only charge (electrostatic interactions), but also steric (constitution) and configurational (D-, L-alanine) contributions affect the ligand-binding site interaction. These studies present further evidence that the route of deiodination is dependent on properties of the ligand-binding site and/or catalytically-active site of the enzyme, and not on the pKa-value of the 4'-OH-group of the ITH-analogue ligands. Alterations of incubation-pH change the dissociation state of the thiolate-imidazolium ion-pair of the enzyme ligand-binding site. This can provoke changes in the enzymic route of the T4-monodeiodination cascade from 5'(3')- to the 5(3)-deiodination pathway and vice versa. The same shift can be obtained by the choice of the configuration of the alanine side chain. ITH-D exhibits substrate (ligand) binding characteristics similar to both TBPA and the nuclear T3-receptor with respect to the ITH-analogue side chain constitution: All three prefer acetic acid derivatives as ligands. In contrast to the nuclear T3-receptor both ITH-D and TBPA prefer ITH-(analogues) with a 3',5'-disubstitution which yields a dissociated 4'-phenoxi group of the molecule. These similarities may suggest that ITH-binding proteins, ITH-receptors and ITH-metabolizing enzymes may represent a closely related family of proteins. They may possibly be derived from a common ancestral ITH-binding protein. The limited substrate specificity of rat liver ITH-D, which fulfills a major contribution in ITH-metabolism in vivo, may be of physiological relevance for the poorly characterized metabolism of naturally occurring (Tetrac) and pharmacologically important (D-T4) ITH-analogs.