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Synthesis and Degradation of Mixed Proteins, and Composition Changes in Skeletal Muscle of Malnourished and Refed Rats^1,2,

Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Selected compositional parameters of hind leg skeletal muscle and the rates of synthesis and degradation of mixed proteins were measured in rats (154 g initial weight) after ad libitum feeding a low protein diet for 17 weeks, and after 2 to 9 days and 59 to 66 days of refeeding with an adequate diet. The half-life for degradation of the mixed muscle proteins was determined from the change in total hind leg radioactivity between 2 and 7 days following an intravenous injection of 4-¹⁴C-aspartic acid. From these data and the size of the hind leg muscle protein pool, the rate of muscle protein synthesis was calculated. The prolonged protein-depletion phase resulted in loss of body weight, increase in the water, chloride and DNA concentrations and a decrease in lipid and RNA in muscle. Refeeding rapidly reversed these changes. The half-life for catabolism of mixed proteins in muscle of depleted rats was 10.2 days as compared (P > 0.1) with a value of 13.1 days for well-nourished weight-control rats (74 g). During the first 9 days of refeeding, the synthesis of mixed muscle proteins proceeded at a greater rate (P < 0.01) than in the weight control rats. In contrast, the catabolism of muscle protein was abolished during this period. After 2 months of refeeding, the half-life for catabolism of mixed muscle proteins was 12.6 days. It is concluded that increased synthesis and decreased catabolism of protein play major roles in bringing about a rapid accumulation of muscle protein during early protein refeeding.

² Contribution no. 1717 from the Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts.

³ On sabbatical leave from Division of Animal Science, The University of Manitoba, Winnipeg, Canada.

Manuscript received 22 January 1971.