A Photoactivated Flavin-induced Degradation of Thyroxine and Related Phenols

Abstract

Previous experiments had revealed that deiodination of thyroid hormones occurred in preparations of frog tissues supplemented with flavin compounds, and that this effect was especially marked in preboiled specimens. The present study was therefore undertaken to investigate a possible direct chemical interaction between flavin derivatives and iodinated phenols. Marked degradation of thyroxine (D- and L-isomers), triiodothyronine, tetra- and triiodothyroacetic acids and mono- and diiodotyrosine occurred when these compounds were incubated in a simple buffer medium (KRP) in the presence of flavin compounds and bright light. This degradation proceeded less rapidly in ambient light and was completely prevented by incubation in either total darkness or in red light. The photoactivated flavin-induced degradation of thyroxine was inhibited by anoxia, and by human plasma (provided this was added to the system before the flavin). The rate of the reaction was reduced in the presence of catalase. Certain hydroxylated derivatives of tryptophane, previously found to inhibit the deiodination of thyroxine by tissue preparations not supplemented with flavin, prevented this photoactivated degradation of thyroxine when they were added to the system before the flavin. A similar photoactivated flavin-induced effect also occurred when biological material was present. In tissue systems which were unable to deiodinate thyroxine in the absence of added flavin, marked degradation was observed in the presence of flavin and bright light. Supplementation of tissue preparations which were capable of deiodinating thyroxine in the absence of added flavin resulted in an enhanced degradation of thyroxine. In all instances, activation by flavin compounds was completely prevented by incubation in the dark. In both tissue and tissue-free systems, 2 major I¹³¹-labeled products were formed: inorganic iodide and a material which remained at the point of application during chromatographic and electrophoretic analysis. The intensity of the photoactivated flavin-induced reaction was dependent on the degree of illumination, the concentration of flavin and the pH of the buffer medium.