Incubation of hepatic microsomes with 3, 5, 3′-triiodo-L-thyronine or L-thyroxine labeled with 126I in the phenolic ring results in both deiodination and the formation of labeled microsomal iodoprotein which cannot be extracted with ethanol. Chromatography of Na3PO4 hydrolysates of the iodoprotein indicated that at least 20% of the radioactivity was in the form of the labeled hormone used in the incubation. Control studies with 131I-labeled hormone, which was added after completion of the incubation, indicate that the 125I-hormone recovered greatly exceeded that which could be attributed to incomplete extraction of unreacted hormone. In addition to the iodothyxonines, iodide and unidentified peaks which remained near the point of application were identified in the digests of the microsomal iodoprotein. Kinetic studies and studies using chemical inhibitors suggest that the rate of hormone-protein complex formation is proportional to the rate of deiodination. The kinetic studies also militate against the possibility that the hormone-protein complex serves as the precursor of the iodide liberated into the medium. A reactive iodothyronine intermediate is postulated which can either be deiodinated or enter into irreversible linkage with microsomal protein. Elucidation of the mechanism of formation of these complexes may provide insight into the processes which lead to similar hormone-protein complexes in vivo.
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