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Accumulation and Turnover of Free and Membrane-Bound Ribosomes in Rat Liver during Short-Term Protein Malnutrition¹

Institute of Human Nutrition, College of Physicians and Surgeons, Columbia University, 630 West 168th Street, New York, New York 10032

Since regulation of ribosome synthesis must involve the transfer of information from both nucleus to cytoplasm and from cytoplasm to nucleus, we have studied liver cytoplasmic rRNA in rats fed a 6% casein diet for 1 week. In protein restricted rats the release of mature rRNA from the nucleolus was slower than in controls, but rRNA was transported to the cytoplasm at a rate similar to controls. Accumulation of newly synthesized rRNA in malnourished rat liver cytoplasm was 40% lower than in controls. Specific activity of free and membrane bound rRNA was 1.5 times greater in malnourished rat cytoplasm and was attributed to fewer, but more highly labeled ribosomes. The half-life, turnover, and fractional turnover values calculated for control rRNA were 4.4 days, 6.3 days, and 15.8% per day, respectively. The corresponding values for protein restricted liver rRNA were 4.8 days, 6.9 days, and 14.3% per day. The cytoplasmic rRNA content of the protein deficient liver was 56% of the control value. RNA turnover was 51% of the control value and the calculated entry of rRNA into the protein restricted liver cytoplasm was 34% of the control value. These results suggest that nuclear mechanisms are responsible for the decrease in cytoplasm rRNA content during dietary protein restriction.

KEY WORDS: • rRNA accumulation • rRNA turnover • protein malnutrition

¹ Supported in part by National Institutes of Health Research Grant No. HD-06682 and National Foundation Grant No. I-285.

Manuscript received 31 October 1977.