Protein synthesis in pulmonary alveolar macrophages. Source of amino acids for leucyl-tRNA

Extracellular, intracellular and tRNA-bound leucine pools of the adherent pulmonary alveolar macrophage were examined to determine the relationships between them and the precursor for protein synthesis. When cells were cultured in media of various leucine concentrations, the patterns of isotope distribution in intracellular and extracellular leucine did not correlate with the patterns seen in protein-bound leucine. hence, the free leucine pools cannot be used reliably as precursors for calculating rates of protein synthesis. tRNA-bound leucine, however, behaved isotopically as if it were the precursor. Constant synthetic rates were calculated using the tRNA specific activity over a wide range of leucine concentrations. In addition, by measuring the tRNA-bound specific activities of three different amino acids, leucine, valine and phenylalanine, and their respective specific activities in protein, we were able to calculate independently three separate but identical synthetic rates. At physiological amino acid concentrations, the macrophage intracellular leucine pool and the tRNA-bound leucine pool received less than half of their amino acids from extracellular sources. At 5 mM external leucine, the intracellular specific activity was indistinguishable from that of the medium leucine, but the specific activity of the tRNA-bound leucine pool remained only about 50% that of the extracellular value. The most straightforward interpretation of why the tRNA-bound leucine did not flood with external label under conditions where the intracellular pool has reached equilibrium is to propose that some portion of the leucine for protein synthesis is derived directly from protein turnover before the degradation products have mixed with the common amino acid pool.