An unusual property of thyroxine and other iodophenols

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Abstract
When solutions of I131-labeled L-thyroxine and related iodophenols (3,5,3[image]-tri-iodo-L-thyronine, 3,5-di-iodo-L-tyrosine, 3-iodo-L-tyrosine and 3,5-di-iodo-p-cresol) in 15-50% aqueous organic solvents were introduced into a large volume of an aqueous buffer, there occurred a very rapid loss of a part of the radioactivity present in the iodophenol. This disappearance was followed by a gradual reincorporation of the radioactivity. The rates of disappearance and re-formation of the radioactive iodophenol were determined by the pH of the aqueous medium and the presence of light. The concentration of the iodophenol, the composition of the buffer and temperature had slight or no effect. The initial reaction of the loss of iodophenol radioactivity was associated with ionization of the phenolic hydroxyl group in these compounds. The cause of the re-formation of the iodinated phenol was not discovered. Although paper-chromato-graphic analysis indicated the formation of a radioactive substance of an RF similar to that of iodide, during the initial reaction, results of other tests were incompatible with a mechanism of deibdination. With L-thyroxine, simultaneous chromatographic and spectrophoto-metric analysis showed a true increase in the concentration of the original substance during the reincorporation of I131 into thyroxine. The hitherto unreported UV-absorption maxima at 231 and 227 mu for L-thyroxine and 3,5,3[image]-tri-iodo-L-thyronine respectively have been described.