Binding of triiodothyronine by bullfrog tadpole liver and tail fin cytosol proteins

Abstract

The binding of triiodothyronine by Rana catesbeiana tadpole liver and tail fin cytoplasmic proteins was studied using protamine sulfate to separate bound and free hormone. The number of binding sites and the dissociation constant for triiodothyronine in liver cytosol did not differ between tissue taken from animals of stage I to V (premetamorphic) and tissue from tadpoles of stage XX (metamorphic climax). The same was true for the binding of triiodothyronine to tail fin cytosol from animals of the two stages. There were, however, significant differences between the liver and tail fin cytosol of tissue from animals of stage I to V for the number of binding sites for triiodothyronine (2.0 × 10 −11 mol/mg protein vs. 6.2 × 10 −11 mol/mg protein) and the dissociation constant (4.6 × 10 −8 M vs. 1.6 × 10 −8 M). Similarly, significant differences between liver and tail fin cytosol from tadpoles of stage XX in the number of binding sites (1.9 × 10 −11 mol/mg protein vs. 7.6 × 10 −11 mol/mg protein) and dissociation constant (3.0 × 10 −8 M vs. 1.3 × 10 −8 M) were observed. No distinctive differences were noted in the rate of dissociation of triiodothyronine from cytoplasmic proteins from the two tissues ( t 1 2 from liver cytosol of 31.5 min vs. t 1 2 from tail fin cytosol of 24.0 min) or in the relative binding of various thyroid hormone analogs. Of interest was the observation that L-thyroxine is a 1000-fold weaker competitor than L-triiodothyronine for displacement of labeled triidothyronine in view of the observation that thyroxine is only a 10-fold weaker inducer of metamorphosis.