Letter | Published:

Increase in binding capacity for triiodothyronine in tadpole tail nuclei during metamorphosis


REGRESSION of the tadpole tail is under direct control of the thyroid hormones and provides a system for study of their mechanism of action1. In this system triiodothyronine (T3) is two to five times as active as thyroxine (T4) in causing tail regression2,3. Studies of the binding of 125I-T3 and 125I-T4 after incubation of hormone with sliced tailfin tissues have revealed high-affinity, saturable binding sites in the cell nucleus4—a maximum of 1,500 and 800 sites per nucleus for T3 and T4, respectively. (Similar results have been obtained with tadpole liver cells5.) The dissociation constants for T3 and T4 were almost identical (10−10 M). These results suggest that in amphibians the maximum number of binding sites rather than the affinity constant correlate with the biological activity of T3 and T4. In mammals, however, specific high-affinity binding sites for the thyroid hormones were found in nuclei6–8 and correlate with the affinity constant.

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