EFFECT OF GROWTH HORMONE ON PROTEIN SYNTHESIS IN ABSENCE OF PEPTIDE CHAIN INITIATION

Pactamycin and dextran sulphate were tested for their ability of inhibit, specifically, peptide chain initiation in a liver cell-free system. The incorporating ability of ribosomes from normal, hypophysectomized and growth-hormone-treated rats was then assayed in the absence of peptide chain initiation. 1. 1. Pactamycin inhibited protein synthesis but not, specifically, the initiation step. 2. 2. Dextran sulphate inhibited the poly U-stimulated incorporation of phenylalanine when added to ribosomes before the polymer. It did not inhibit incorporation in the absence of poly U if added after incubation had commenced. It was concluded that dextran sulphate specifically inhibited chain initiation. 3. 3. Dextran sulphate inhibited the basal level of incorporation when added to ribosomes before cell sap and other factors. It was concluded that some of the incorporation observed in the liver cell-free system results from initiation of peptide chain synthesis, but that this occurs only in the first moments of incubation. 4. 4. The differences in incorporating ability of liver ribosomes of normal, hypophysectomized and growth-hormone-treated rats persisted even when the initiation of peptide chain synthesis had been inhibited by dextran sulphate. It is suggested that growth hormone may act to increase the ratio of active to inactive ribosomes. © 1969.