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Protein Deficiency: Turnover of Protein and Reutilization of Amino Acid in Cell Fractions of Rat Liver¹

Department of Pediatrics, University of California-San Francisco, San Francisco, California 94122

The effect of a protein-deficient diet on average turnover of protein in subcellular fractions of the liver was studied in rapidly growing young and fully grown adult rats. Young (28 days) and adult (> 200 days) female rats were fed diets containing 3.4% or 26% protein. After 3 and 14 days, respectively, they were given an intraperitoneal injection of ¹⁴C-guanidino-arginine and (4-³H)-arginine. The former label is less subject to reutilization than the latter and should therefore provide more accurate turnover data. Groups of rats were killed 1, 2, 3, and 5 days after pulse-labeling. Mitochondrial, microsomal, microsomal membrane, ribosomal, and supernatant fractions were prepared from the liver and analyzed for acid-precipitable protein and radioactivity. Our results indicate little or no difference in the average rates of ¹⁴C-protein degradation between young and adult rats fed the 26% protein diet. There was a large discrepancy between the rates of ¹⁴C and ³H decay only in the young animals. This indicates a high rate of arginine reutilization for protein synthesis. The 3.4% protein diet in both age groups resulted in either equivalent or increased rates of protein degradation compared with the 26% protein group. Thus, when errors due to recycling were reduced by the use of ¹⁴C-guanidino-arginine, there was no evidence of a decrease in the average turnover of liver protein as a result of protein deficiency.

KEY WORDS: • protein deficiency • protein turnover • subcellular fractions • ¹⁴C-guanidino-arginine

¹ Supported by grants from the USPHS (AM 13897 and HD 05875) and the John A. Hartford Foundation. Dr. Dallman is the recipient of a USPHS Career Program Award, HE 31766.

Manuscript received 29 June 1972.