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 depressed2,3 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 mRNA2,4, 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.