Antihormonal Effects of Plant Extracts: Iodothyronine Deiodinase of Rat Liver is Inhibited by Extracts and Secondary Metabolites of Plants

Abstract

1. Aqueous extracts from plants such as Lycopus virginicus, Melissa officinalis (Laminaceae), and Lithospermum officinale (Boraginaceae), containing various antihormonal components, also inhibit both the extrathyroidal enzymic T₄-5′-deiodination to T₃ and the T₄-5′-deiodination. The effects were dose dependent and ultimately complete using rat liver microsomes as a source of the enzyme in vitro. 2. The “specific inhibitory activity” of extracts depends on the plant species used and the extraction procedure applied. It can be increased by ether extraction of freeze dried aqueous extracts and decreased by oxidation with KMnO₄. 3. The active principle(s) exhibits chemical characteristics of phenols or phenolcarboxylic acids: rosmarinic acid, ellagic acid, and luteolin-7β-glucoside are active inhibitory components whilst other secondary plant metabolites consisting only of a single diphenolic ring e.g. cinnamic acid and derivatives are inactive. 4. The inhibition of both iodothyronine-5′- and -5-deiodinase by this new class of iodine-free phenolic inhibitors - belonging to the cinnamic acid-flavonoid-type - supports the hypothesis that in rat liver only a single enzyme catalyses both deiodination pathways and might be of pharmacological interest for the treatment of hyperthyroidism.