Demonstration of a Membrane-Linked Deiodination of Thyroid Hormones from Human Red Cell Membranes

Abstract

The deiodinaton of ¹³¹I-labeled thyroxine and triiodothyronine by red cell membranes prepared from healthy persons was examined. Under the experimental conditions (pH 6.0, 32°C) 0.5 mg membranes deiodinated 1.6 mµ moles triiodothyronine in 30 min. Iodide and a substance with an R_f of 0 in paper chromatography were the only radioactive deiodination products. The membrane system was shown to be heat stabile and dependent on an alkali labile nondialyzable factor. Iodide release is linked to membrane particles and can be demonstrated even after lipid extraction of the ghosts. The deiodinating system is highly active without the addition of any activators and is characterized by time dependency and a pH optimum of 6.0. Stimulators of the reaction are oxygen, Ca²⁺, Cu²⁺ and fructose 1,6-diphosphate. A peroxidative mechanism catalyzed by traces of hemoglobin and a non-enzymatic FMN light-induced iodide release could be excluded. The membrane-linked deiodination might play an important role in the peripheral metabolism of thyroid hormones.

Key words

Deiodination - Thyroid Hormones - Red Cell Membranes - Thyroxine Metabolism