Tiie Joutrnal of Bioloqical Chemistry

The deiodination of labeled L-thyroxine and L-triiodothyronine (T3) in the presence of flavin mononucleotide, light, oxygen, and phosphate buffer, pH ‘7.4, was followed by measuring the iodide released. Under standardized conditions the amount of deiodination was found to be proportional to light intensity. At any T3 concentration, the reaction is first order with respect to Tat whereas FMN acts catalytically. No evidence was found for the existence of an FMN-Lthyroxine or FMN-iodine complex. The reaction velocity of the deiodination of T3 was found to be proportional to the concentrations of FMN and T3 up to a concentration of FMN of 2 X lop5 M. The activation energy for the deiodination of L-thyroxine is 8900 cal. Iodide and increasing ionic strength inhibited the reaction. Amino acids, amines, proteins, and some metal chelators were found to accelerate the FMN-induced deiodination. The stimulatory effect of aliphatic amino acids, ribonuclease, and ethylenediaminetetraacetate disappeared when the system was made metal-free by means of Dowex A-l and by extraction with dithizone in heptane. l?e2+, Zn2+, C&, and Cazf, in concentrations of 5 x 10e8 M, added to the purified system restored the stimulatory activity of these agents. Different metal ions and different concentrations were found necessary for optimal effects depending on the amino acid or proteins employed. Fez+ plus EDTA, on the other hand, was inhibitory at concentrations where Fez+ and EDTA alone did not influence deiodination. Cupferron, quinoline-8carboxylic acid, and 8-hydroxyquinoline inhibited deiodination in the ordinary system. Tryptophan, histidine, and tyrosine showed no metal requirement. The accelerated deiodination of L-thyroxine by aliphatic amino acid and proteins, as well as inhibited deiodination by chelators plus metal ions, is considered to be due to chelation with bivalent metal ions.