THE METABOLIC INTERRELATIONS OF THYROXINE AND DIIODOTYROSINE IN THE THYROID GLAND AS SHOWN BY A STUDY OF THEIR SPECIFIC ACTIVITY-TIME RELATIONS IN RATS INJECTED WITH RADIOACTIVE IODINE

The evidence in support of the hypothesis that thyroxine is formed in the thyroid from tyrosine with diiodotyrosine as an intermediate has recently been reviewed by Harington (1).The presence of relatively large amounts of diiodotyrosine in the thyroid gland and, in particular, t,he finding that natural thyroxine and natural tyrosine are configuratively related are considered by hii to favor strongly such a mechanism.Further support is offered by the isolation of thyroxine from iodinated proteins (2-4) and by the finding that thyroxine is formed during the prolonged incubation of diiodotyrosine in a slightly alkaline solution (5-10).Zilversmit et al. have devised a procedure based on the use of labeling agents for testing whether one compound in a tissue is a precursor of another ( 11).This involves a study of changes in the specific activities of the two compounds with time.With the aid of methods recently developed in this laboratory for fractionating the iodine in as little as 20 mg. of thyroid tissue (12) and with the aid of radioactive iodine, it became possible to apply this procedure to a study of the interrelations of diiodotyrosine and thyroxine in the thyroid glands of the rat.EXPERIMENTAL Rats of the Slonaker and Curtis-Dunning strains weighing 180 to 250 gm. were used in this study.All had been raised on the same diet.Each rat received by way of the tail vein 0.5 to 1.0 cc. of an isotonic solution containing 20 microcuries of P per cc.with no 1121 carrier.The rats were sacrificed in groups of three or four at the following intervals thereafter: 15 minutes, 1 hour, 4, 14, and 50 hours.Fractionation of Iodine-The rats were anesthetized with nembutal, bled by severing the heart and great vessels, and their thyroids removed.The glands were then rapidly weighed and homogenized with 1 cc. of cold 10 per cent trichloroacetic acid in a small glass homogenizer.The homogenate together with the water washings used in its preparation