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Long-term Protein Deficiency and Rat Liver Ribosome Cycle¹

Istituto Nazionale della Nutrizione—Via G. M. Lancisi, 29. 00161—Rome, Italy

The turnover of free and membrane-bound liver ribosomes has been studied in 150 g male rats fed a protein free diet for 30 days and compared to that of control rats at the beginning of the experiment. The incorporation of ¹⁴C-orotate into ribosomal RNA 1, 2, 4, and 6 hours after intraperitoneal injection was studied separately in polysomal ribosomes and in single ribosomes not engaged in protein synthesis. In control rats, the rate of ¹⁴C-orotate incorporation into RNA of polysomal ribosomes was similar for free and membrane-bound ribosomes, while in protein deprived ones, a slightly more rapid incorporation of the precursor into membrane-bound than into free polysomal ribosomes was observed. Moreover, for protein deprived rats, the rate of ¹⁴C-orotate incorporation into RNA was higher for both ribosomal populations than that of control rats. The specific activity of the RNA precursor pool (5'-UMP) after 6 to 360 minutes labeling time was slightly lower in protein deprived than in control rats and therefore does not account for the higher rate of labeling of structural RNA of ribosomes of deprived rats. The specific activity of single ribosome RNA after 6 hours of labeling, in control rats was not different from that of polysomal structural RNA; while in protein deprived rats, it was still significantly lower. This last finding supports the hypothesis that single ribosomes accumulated in protein deficiency represent an inactive reserve pool.

KEY WORDS: • protein deficiency • liver ribosome cycle • RNA synthesis rate

¹ Presented in part at the ninth FEBS meeting Budapest, Hungary 1974.

² Present address: Istituto Fisiologia Generale—Università, 00185 Rome, Italy.

Manuscript received 2 June 1976.