1. Rates of synthesis and catabolism of liver and muscle sarcoplasmic and myofibrillar protein have been measured in young control, starved and protein (deprived) rats using [14C]Na2CO3 to label protein.

2. Half-lives for synthesis of 1·35, 2·8 and 7·2 days for liver, sarcoplasmic and myofibrillar proteins respectively were obtained, whilst half-lives for catabolism were 1·55, 3·6 and 15·6 days in each case in the control animals.

3. The protein free diet for 3 days caused a small decrease in the rate of synthesis of liver and muscle proteins. The catabolic rate of liver protein was increased by 20% whilst there was a smaller increase in the catabolic rate of myofibrillar proteins.

4. Starvation for 3 days caused a 20% reduction in the rate of liver protein synthesis whilst there were greater reductions in muscle protein synthesis. The catabolic rate of liver protein was only slightly increased whereas there was a 75% increase in the rate of myofibrillar protein breakdown.

5. The total amount of protein synthesis and catabolism in liver and the two muscle protein fractions over the first 3 days of the three regimes were calculated. Muscle protein turnover, in particular myofibrillar, was shown to be very sensitive to dietary protein and/or calorie deficiency.

6. These results are discussed in terms of the mobility and therefore importance of muscle protein metabolism in the economy of the whole animal.

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