Effects of Thyroxine on Initiation and Completion of Protein Chains of Hemoglobin in Vitro

Abstract

Abstract Thyroxine added to a rabbit reticulocyte lysate system in vitro stimulates amino acid incorporation into the lysate protein. In agreement with previous findings in other cell-free preparations, the thyroxine effect is dependent on the addition of mitochondria and an oxidizable substrate to the reaction system. The products of the thyroxine-stimulated amino acid-incorporation reaction include both the α and β chains of hemoglobin. By analysis of both the amino-terminal and the interior valine positions labeled with l-valine-14C during polypeptide chain formation, it was found that the thyroxine stimulation of β chain synthesis involves both chain initiation and completion, i.e. complete synthesis de novo, although the effect on completion is considerably greater than that on initiation. In the case of α chain synthesis, thyroxine stimulated the completion of chains previously begun in vivo without any apparent effect on the initiation of new chains during the incubation. The data suggest that the primary site of the thyroxine stimulation of protein biosynthesis is at a late stage in polypeptide chain elongation, chain termination, or chain release, and that chain initiation may be secondarily increased when the rates of the earlier reactions are favorable.