The distribution of ribosomes between different functional states in livers of fed and starved mice

Abstract

1. To investigate the role of ribosome function in regulating protein synthesis, the activity, distribution and functional states of ribosomal particles were investigated in livers of mice fed ad libitum or starved overnight. 2. The distribution of protein-synthesizing activity between polyribosomes of different sizes was analysed after incorporation of radioactive leucine, and the quantitative distribution of ribosomes as native subunits, monomers and polyribosomes was analysed after incorporation of orotic acid. Precursors labelled with (3)H or (14)C were given separately to fed and starved mice, so that livers from the two groups of animals were processed together. 3. The former experiments showed that starvation has little effect on the distribution of protein-synthesizing activity across polyribosome sedimentation patterns, though the latter experiments showed that the proportion of ribosomes existing as monomers increased from 9.5% to 15.2%, whereas the proportion existing as polyribosomes decreased from 81.4% to 75.6%. Starvation had a negligible effect on the proportion of native subunits, which accounted for 9.1% and 9.2% of the ribosomes in fed and starved mice respectively. 4. The monomeric ribosome fraction was isolated and subjected to ionic conditions which selectively dissociate single ribosomes. Starvation increased the proportion of monomers that dissociated from 59% to 72%, so the monomers that accumulate in livers of starved animals are single ribosomes and not monoribosomes resulting from degradation of polyribosomes. 5. The fate of newly formed ribosomal particles was studied by measuring the specific radioactivity of native subunits, monomers and polyribosomes at different times after injection of radioactively labelled orotic acid. Starvation did not appear to affect equilibration between newly formed particles and polyribosomes, and the radioactivity of polyribosomes in both groups of mice reached about 90% of that in native subunits after 4h. The radioactive labelling of monomers proceeded at a slower rate, especially after starvation. At 4h, the radioactivity of monomers was 64% and 55% that of native subunits in fed and starved mice respectively.