PHENYLTHIOUREA TREATMENT AND BINDING OF RADIOACTIVE IODINE IN THE TADPOLE

¹ Biology Division, Oak Ridge National Laboratory, and Departments of Biology, The Catholic University of America and University of Virginia

1. Larvae of Hyla versicolor were immersed in solutions of phenyithiourea at 0, 4, and 15 days after hatching. At 4, 24, and 39 days after hatching, I¹³¹ was administered and contact autoradiograms were prepared from serially sectioned representative specimens.

2. The tadpoles treated with phenylthiourea from the time of hatching became completely unpigmented. The blanching of the second series was slower and never coniplete. The third series became very little lighter during the course of the investigation. This indicates that the metabolic turnover of melanin goes on at a decreasing rate as the larvae increase in age.

3. From the autoradiograms, evidence was obtained to confirm earlier findings of the binding of iodine in pigmented areas, to show that the binding is apparently not associated with formed melanin, and to support the view that the same enzyme or enzymes that catalyze melanogenesis can catalyze the binding of iodine (presumably with tyrosine).

4. The accumulation of iodine by the horny teeth was inhibited to some degree by phenylthiourea treatment.

5. Accumulation of radioiodine by the thymus gland was confirmed and was found to be completely inhibited by phenylthiourea treatment.

6. The thyroid rudiment acquires the facility for concentrating iodine even before follicle formation begins, and at that time it also responds to the inhibitory action of phenylthiourea.