The effect of triiodothyronine on the transport and incorporation of amino acids by bullfrog tadpole tail fin cells

Abstract

Abstract The effect of thyroid hormones on the transport and incorporation of various amino acids by cells isolated from bullfrog tadpole tail fins has been studied. It was observed that triiodothyronine (T3) produced a rapid inhibition in the transport and incorporation of L-site amino acids. Inhibitory effects were detectable at 10−7 M T3 and averaged over 50% at 10−6 M T3. No effects on A-site amino acid uptake were observed except at 10- to 100-fold greater T3 concentrations. All active thyromimetic compounds with amino acid side chains tested reduced transport and incorporation of L-site amino acids. Thyromimetic compounds with exclusively acid side chains did not inhibit the transport of L- or A-site amino acids. Competition experiments suggested that T3 and L-site amino acids (leucine, phenylalanine) were competing directly for the L-sites, with T3 showing a higher affinity. Inhibitors of protein synthesis, particularly cycloheximide, blocked incorporation with no reduction in transport. No other hormonal effects on amino acid transport or incorporation in tadpole tail fin cells were observed.