Incorporation of Leucine into Human Skeletal Muscle Proteins: A Study of Tissue Amino Acid Pools and their Role in Protein Biosynthesis

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Abstract
The in vitro incorporation of labelled leucine into human skeletal muscle proteins was studied with the aim to elucidate the relationship between the amino acid tissue pools and protein biosynthesis. The distribution volumes of leucine and cycloleucine in skeletal muscle tissue were similar but the equilibration time was shorter for leucine than for cycloleucine. The cellular uptake of leucine and cycloleucine was competitively inhibited by increased concentration of amino acids in the medium indicating an active transport. Optimal stimulation for incorporation of leucine into proteins was obtained at an amino acid concentration in the medium corresponding to 10 times that of normal human plasma. The incorporation of 14C‐leucine into skeletal muscle proteins was linear before the total pool of free intracellular 14C‐leucin had reached an equilibrium. The regression coefficients for the semilogarithmic plots of the labelling rate of proteins with 14C‐leucine and the incorporation rate of leucine calculated from the specific activity in the medium versus the amino acid concentration in the medium were different in the same experiment indicating a re‐utilization of amino acids released at protein degradation. The results are compatible with the hypothesis that the proteolytically released amino acids have a competitive advantage for incorporation as compared with extra‐ and intracellular free amino acids. It is concluded that the amino acid pool which is in the immediate continuity with the protein biosynthesis sites equilibrates rapidly with the extracellular amino acid pool.