Deiodination of Thyroxine by Erythrocytes and Leukocytes from Euthyroid and Hyperthyroid Patients

Abstract

Intact washed erythrocytes and leukocytes from euthyroid and hyperthyroid patients were shown to deiodinate ¹³¹l-L-thyroxine measured by high voltage electrophoresis. Iodide and a substance with an Rf of 0 were the only radioactive deiodination products. Red cells from hyperthyroid patients deiodinated thyroxine significantly faster than cells from euthyroid patients. Leukocytes showed only a minimal deiodination independent of the thyroid state. The reaction in the red cell system requires an intact cell membrane, is stimulated by oxygen and inhibited by mercuric chloride. The deiodinating system is thermolabilc. Its activation energy amounts to 11 900 Cal. With respect to cofactors, this deiodination seems to be quite different from that of other tissues. Since deiodination of thyroxine by red cells can be measured directly in vitro, the red cell system represents a useful tool for further studies of the peripheral metabolism of thyroxine in man. T4 for L-thyroxine; T₃ for L-triiodothyronine; FMN for flavin mononucleotide; FAD for flavin adenine dinucleotide; EDTA for ethylene diamine tetraacetic acid. 1 Supported by Deutsche Forschungsgemeinschaft.