Comparative Effects of Various Analogues of Thyroxine on Amino Acid Incorporation Into Protein

Abstract

The effects of a number of analogues with varying degrees of thyroxinelike biological activity in vivo have been compared with those of L-thyroxine on amino acid incorporation into protein in cell-free rat liver preparations. D-Thyroxine, administered in vivo in doses at which L-thyroxine clearly stimulates amino acid incorporation, is without effect. When added directly to the reaction mixtures containing the cell-free preparations, D-thyroxine is as effective as Lthyroxine in stimulating amino acid incorporation over a range of concentrations. LTriiodothyronine and tetraiodothyroacetic acid stimulate amino acid incorporation after both in vivo and in vitro administration. In vivo they are both at least as active as Lthyroxine. In vitro, however, triiodothyronine has only a fractional part of the activity of L-thyroxine, but tetraiodothyroacetic acid is considerably more active than L-thyroxine, and its optimal concentration is one fourth that of L-thyroxine. The in vitro effect of tetraiodothyroacetic acid exhibits the same 5 min lag period preceding the stimulation of amino acid incorporation observed with Lthyroxine. The results of experiments with miscellaneous analogues indicate the following order of effectiveness in stimulating amino acid incorporation in vitro as regards the degree of iodination: tetraiodo >triiodo>diiodo. The completely deiodinated analogue, thyronine, is without any significant effect. No great differences in activities of analogues with acetic acid and propionic acid side chains were noted; the propionic acid side chain may, perhaps, confer slightly greater activity. (Endocrinology75: 304, 1964)