Inhibition of rat liver iodothyronine deiodinase. Interaction of aurones with the iodothyronine ligand-binding site.

We report that aurone derivatives of plant extracts produce potent, dose-dependent, and ultimately complete inhibition of three different metabolic monodeiodination pathways catalyzed by rat liver microsomal type I iodothyronine deiodinase. These data show that (3'),4',4,6-(tetra)trihydroxyaurones are the most potent naturally occurring plant-derived inhibitors of this deiodinase enzyme (IC50 V 0.5 microM). Lineweaver-Burk analysis using both L-thyroxine (T4) and 3',5',3-triiodothyronine as substrates suggests a cofactor competitive mechanism of inhibition for 4',4,6-trihydroxyaurone which also can displace 125I-L-T4 from binding to thyroxine-binding prealbumin with a potency comparable to its inhibition of T4-5'-deiodinase. Among type I deiodinase inhibitors, cofactor competition has been observed only for propylthiourea. Computer graphic modeling studies were also carried out to explore aurone conformations and to compare them with those of the thyroid hormones. This analysis shows that the aurones can adopt either a planar or an antiskewed conformation, such as observed for 3',5',3-triiodothyronine, the most potent natural deiodinase substrate inhibitor. The thyroxine-binding prealbumin complex was used to model the deiodinase ligand binding site because of the similarity observed between inhibitor binding affinity and enzyme inhibition characteristics. These studies show that the aurones which adopt an antiskewed conformation can interact favorably in the prealbumin binding site. This model of the deiodinase active site can be used to design other deiodinase inhibitors.