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

Abstract

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 regression^2,3. 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 nuclei^6–8 and correlate with the affinity constant.