On the Identification of 4-Hydroxy-3,5-Diiodophenylpyruvic Acid in Rat Thyroid Glands

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Abstract
Recent reports have indicated that 4-hydroxy-3,5-diiodophenylpyruvic acid (DIHPPA) may be an important intermediate in the biosynthesis of thyroxine. This intermediate has not, however, been identified conclusively in thyroid tissue. Extracts of rat thyroid glands labeled with iodide-125I in vivo were chromatographed on thin-layer silica gel sheets in 2 solvent systems. Radioautographs of these chromatograms showed radioactivity in the DIHPPA and other areas. Attempts to purify this compound by elution and rechromatography proved unsuccessful. It was shown by ultraviolet spectroscopy that significant structural alterations occurred during these procedures. The resulting compound was tentatively identified as 4-hydroxy-3,5-diiodophenylacetic acid. In other studies, prelabeled rat thyroids were homogenized in hot glacial acetic acid containing 100 mg unlabeled DIHPPA and 2 mg Nal. The carrier was recrystallized to constant specific activity. After 5 crystallizations the methyl ester of DIHPPA was synthesized using diazomethane. It too was recrystallized to constant specific activity from 50% ethanol. Control studies were performed in which DIHPPA was recrystallized in the presence of 2 mg Nal and radioactivity in the form of iodide-125l thyroxine-125I or diiodotyrosine-125I. The added radioactivity was 5-10 times the estimated cpm present as the free compounds in the thyroid. In all cases the specific activity of DIHPPA attained was less than 1/10 that of the experiments where the thyroid was labeled endogenously. The studies reported here demonstrate that DIHPPA is present in rat thyroid glands representing approximately 0.03% of the thyroidal radioactivity. The physiological importance of this compound remains to be evaluated.

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