Evidence That the Intracellular Pool of Tyrosine Serves as Precursor for Protein Synthesis in Muscle

Several investigators suggested that labeled amino acids are incorporated into protein without first equilibrating with the intracellular pool. Experiments were performed to determine if the intracellular or extracellular [14C]tyrosine represented the precursor for protein synthesis in muscle. Four pieces of rat diaphragm were incubated in the same concentration of [3H]leucine but in different concentrations of [14C]tyrosine. The rates of [3H]leucine incorporation were similar in all the muscles, indicating similar rates of protein synthesis. The 14C counts in protein increased with increasing tyrosine concentrations in the medium, but the moles of tyrosine incorporated into protein were similar only when the counts in protein were divided by the intracellular specific activity. In other experiments muscles were incubated with [3H]leucine and [14C]tyrosine, and then transferred to medium containing [3H]leucine and unlabeled tyrosine. The rate of tyrosine incorporation agreed with leucine incorporation only when calculations used the intracellular tyrosine specific activity. Therefore, the free intracellular tyrosine resembled a precursor for protein synthesis, while the extracellular did not. However, this conclusion did not hold during very short incubations or incubations with low concentrations of tyrosine in the medium, for reasons suggested herein.