Protein Feeding of Rats after Protein Starvation: Incorporation of Amino Acid into Polypeptide by Skeletal Muscle Polyribosomes

Abstract

Female rats of about 100 g body weight were fed a protein-free diet for several days. The amino acid incorporating activity of cell-free preparations from their skeletal muscle was compared with that of rats which had received an isocaloric meal of adequate protein after protein starvation. The source of soluble enzymes was the liver from rats kept on a normal diet. Both microsomal and ribosomal preparations showed a decrease in amino acid incorporation after protein starvation for 4 to 6 days. The activity was enhanced by giving a single meal of protein. The enhancement was more pronounced with ribosomal preparations (35 to 60%) as compared with microsomal preparations (15 to 40%). The polysomal pattern was unaffected by the dietary alterations. Incubation of the cell-free preparations with cell sap devoid of low molecular weight components increased the ability of the microsomes and ribosomes to incorporate amino acids into protein. As judged from the amount of monoribosomes present after incubation with this type of cell sap it seemed that the monosomes were reutilized for polypeptide synthesis. It is concluded that, although the activity of the systems is decreased after protein starvation of the animals, the functional sites for polypeptide formation are fully intact.