Rat liver iodothyronine monodeiodinase. Evaluation of the iodothyronine ligand-binding site.

Ligand binding characteristics of rat liver microsomal type I iodothyronine deiodinase were evaluated by measuring dose-response inhibition and apparent Michaelis-Menten or inhibitor constants of iodothyronine analogues to compete as substrates or inhibitors for the natural substrate L-thyroxine. These data show strong correlations with the binding requirements of hormone analogues to serum thyroxine-binding prealbumin since iodothyronine analogues with a negatively charged side chain, a negative charge or hydrogen bonding function in the 4'-position, tetraiodo ring substitution, and a skewed hormone conformation are structural features shared in common which markedly affect enzyme activity and protein binding affinity. 3,3',5'-Triiodo-L-thyronine is the most potent natural substrate (IC50 = 0.3 microM) and tetraiodothyroacetic acid is the most potent inhibitor (IC50 = 0.2 microM). Both thyroxine (T4)-5'- and T4-5-deiodination pathways are inhibited by these potent analogues, providing further evidence for a single enzyme catalyzing the rat liver microsomal deiodination reactions. These data also show that L-hormone analogues are preferentially deiodinated via the T4-5'-deiodination pathway, whereas D-analogues produce products via the T4-5-deiodination pathway. The thyroxine-binding prealbumin complex was used to model the interaction of thyroid hormones with the deiodinase active site. Computer graphic modeling of the prealbumin complex showed that only those analogues which are potent deiodinase inhibitors or substrates can be accommodated in the hormone binding site. This model suggests the design of functionally specific ligands which can modulate peripheral thyroid hormone metabolism and act as antithyroidal drugs.