TRANSMEMBRANE TRANSPORT AND INTRACELLULAR KINETICS OF AMINO-ACIDS IN HUMAN SKELETAL-MUSCLE

We have used stable isotopic tracers of amino acids to measure in vivo transmembrane transport of phenylalanine, leucine, lysine, alanine, a nd glutamine as well as the rates of intracellular amino acid appearan ce from proteolysis, de novo synthesis, and disappearance to protein s ynthesis in human skeletal muscle. Calculations were based on data obt ained by the arteriovenous catheterization of the femoral vessels and muscle biopsy. We found that the fractional contribution of transport from the bloodstream to the total intracellular amino acid appearance depends on the individual amino acid, varying between 0.63 +/- 0.02 fo r phenylalanine and 0.22 +/- 0.02 for alanine. Rates of alanine and gl utamine de novo synthesis were approximately eight and five times thei r rate of appearance from protein breakdown, respectively. The model-d erived rate of protein synthesis was highly correlated with the same v alue calculated by means of the tracer incorporation technique. Furthe rmore, amino acid transport rates were in the range expected from lite rature values. Consequently, we conclude that our new model provides a valid means of quantifying the important aspects of protein synthesis , breakdown, and amino acid transport in human subjects.