Turnover of the sarcoplasmic proteins aldolase, phosphoglycerate mutase, lactate dehydrogenase and creatine phosphokinase isolated from chicken breast muscle was investigated using a pulse labelling technique. 2. A single injection of [U-14C]leucine was given and the proteins were extracted and purified at 2, 6, 15, 30, 48 and 72 hr following administration. Specific radioactivity in all of these isolated enzymes showed unexpected multiple peak profiles which did not intersect with the specific radioactivity profile of the blood plasma. 3. These results were interpreted as showing that either a large proportion of these proteins was not turned over in rapidly growing muscle or that the plasma amino acid pool was not the precursor pool for muscle protein synthesis. 4. The results also suggested that at least two sub-populations of the proteins exist within the muscle tissue. 5. A further conclusion drawn from these data was that established techniques of pulse labelling may seriously overestimate the rate of protein synthesis in growing muscle.
