Observations on the iodine-containing compounds of the thyroid gland. Isolation of <i>dl</i>-3:5-di-iodotyrosine

Abstract

ABOUT three years ago one of the authors [Harington, 1926] described an improved method for the isolation of thyroxine from the thyroid gland, which was based on a graduated hydrolysis of the gland substance with barium hydroxide. When desiccated thyroid was boiled with dilute barium hydroxide the iodine was separated into three fractions, the first being contained in the precipitate of insoluble barium salts, the second being obtainable by acidification of the alkaline filtrate, and the third remaining soluble in presence of acid. Thyroxine was isolated by subjecting the first two fractions to more intensive hydrolysis with barium hydroxide, whilst no evidence could be obtained of the presence of thyroxine in the third, acid-soluble, portion. A similar separation of the iodine compounds of the thyroid into acid-soluble and acid-insoluble had been effected previously by Kendall by hydrolysis with dilute sodium hydroxide, and he was able to show [1919] that the physiological activity was confined to the acid-insoluble fraction, i.e. to that fraction from which alone thyroxine could be isolated. This observation has been confirmed by an experiment kindly performed for us by Dr J. H. Gaddum. Desiccated thyroid gland was boiled with 10 % crystalline barium hydroxide under the usual conditions and the hydrolytic.products were separated into acid-insoluble and acid-soluble fractions; injection of 2'6 mg. per kg. of the iodine of the acid-insoluble portion produced an increase of 60 % in the oxygen consumption of a rat, whilst a definitely smaller effect was obtained after injection of 26 mg. per kg. of the iodine of the acid-soluble fraction. It is evident, therefore, that this preliminary hydrolysis, although it is of so mild a character that it cannot conceivably involve any destruction of thyroxine, effects a practically complete separation of the latter from the other iodine-containing compounds which may be present.