Impaired Ability of Ribosomes to react with Polyuridylic Acid after Hypophysectomy of the Rat : Removal of the Disability by a Cell Sap Factor

Abstract

THE liver ribosomes of hypophysectomized rats are less able to incorporate amino-acids into protein in vitro than those of normal rat liver¹. Ribosomal preparations from hypophysectomized rats contain less polysomes than those from normal rats², and the rate of synthesis of messenger ribonucleic acid (mRNA) and other kinds of RNA is depressed^2,3 after hypophysectomy. The action of growth hormone in promoting protein synthesis and growth can be explained in terms of its action in stimulating the synthesis of mRNA^2,4, but this hypothesis does not explain all the facts. It has been noted⁴, for example, that the percentage stimulation of phenylalanine incorporation obtained by addition of 25 µg of polyuridylic acid (poly U) to the cell-free system with ribosomes from liver of hypophysectomized rats was as great as, or greater than, that obtained when ribosomes from liver of normal rats were used; but the absolute level of incorporation with ribosomes from hypophysectomized rats was still less than that obtained with normal rat liver ribosomes. The amount of poly U used was arbitrary and was probably not optimal for the system; thus it was decided to reinvestigate this point under conditions which were optimal for incorporation of phenylalanine into polypeptide in the presence of poly U.