1. The turnover of rat skeletal muscle protein was studied using [75Se]selenomethionine, [6-14C]arginine and [14C]Na2CO3 to label protein.

2. In rats labelled with both [75Se]selenomethionine and [14C]Na2CO3 the 14C activity of mixed skeletal muscle protein fell rapidly with a half-life of 6·0 days for the specific activity and 10·5 days for the total activity. There was no loss of 75Se activity from muscle protein during the 12 days of the experiment.

3. Following the injection of [6-14C]arginine both sarcoplasmic and myofibrillar proteins continued to incorporate label for 6 days after which time the label was lost fairly rapidly.

4. Following the injection of [14C]Na2CO3 muscle protein was maximally labelled by 6 h, at which time specific activity of the free amino acids had fallen to a very low level. Aspartate and glutamate in particular had lost over 99% of their maximum activity by this time in comparison to arginine which was still highly labelled after 24 h.

5. 14C activity was lost more rapidly from aspartate and glutamate isolated from sarcoplasmic and myofibrillar protein than from the other labelled amino acids. The half-lives of the two protein fractions were 3·9 and 7·2 days from the specific activity curves and 6·0 and 19·0 days from the total activity curves.

6. The differences between the half lives of muscle proteins labelled with different amino acids are discussed in terms of the effects of reutilization of the labelled amino acid used. It is postulated that aspartate and glutamate labelled by the injection of [14C]CO=3 are only reutilized to a very small extent and therefore afford the means by which the rates of protein synthesis and catabolism in skeletal muscle can be measured with reasonable accuracy.

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