Turnover of Liver Ribosomes in Fed and in Fasted Rats

Abstract Evidence from many sources indicates that the ribonucleic acid and protein of normal animal tissues are replaced more rapidly than are the cells themselves. However, the significance of this macromolecular turnover and the mechanisms that regulate synthesis in concert with catabolism to maintain cellular composition have remained obscure. In beginning a study of factors that influence protein and RNA turnover in mammalian cells, we have compared the rates of synthesis and degradation of liver cytoplasmic ribosomes in fed and in fasted animals. Synthesis was measured by labeling ribosomal protein and RNA and by measuring the rate at which the specific activities decreased owing to continuing synthesis from unlabeled precursors. In fed, nearly mature animals, degradation approximates synthesis. In fasting, degradation exceeds synthesis and was measured as the difference between rates of synthesis and of net loss of liver ribosomes. Our results indicate that ribosomal protein and RNA are replaced at the same rate. The normal kinetics are first order, with turnover half-times of 5 days. During starvation, the cellular ribosome content decreases steadily due to an increased degradative and a diminished synthetic rate.