Source of Amino Acids Used for Protein Synthesis in HeLa Cells

HeLa cells, prelabeled with [3H]leucine for several generations, were pulse‐labeled with [14C]‐leucine. The 14C/3H ratio of the leucine in the medium, in the total amino acid pool and bound to transfer RNA was determined. It was found that the 14C/3H ratio of the leucyl‐tRNA was identical with that of the extracellular medium while that of the amino acid pool was much lower. These results confirm previous reports that selection of amino acids for protein synthesis takes place before the amino acids are released intracellularly. Further, it was concluded that amino acids resulting from intracellular proteolysis are not reutilized directly under the conditions of our experiments. A possible model of intracellular amino acid pathways is discussed.