Alterations in Tyrosine and Protein Kinetics Produced by Injury and Branched Chain Amino Acid Administration in Rats

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Abstract
To determine whether the alterations in amino acid metabolism after injury were a result of changes in protein synthesis, whole body tyrosine and individual tissue protein kinetics were estimated 24 h after 3 different forms of stress in rats. In addition, injured rats were starved or infused with 180 mg of N/day (as branched chain amino acids or L-alanine) to ascertain whether the mechanism and degree of N sparing were unique to branched chain amino acid administration or whether they could be attributed to the infusion of .alpha.-amino N. In the more catabolic types of injury, increased N loss during starvation appeared to be due to an increased plasma amino acid appearance and a greater percentage being oxidized. Rates of tyrosine incorporation into whole body protein were also enhanced and could be explained in part by increases in the fractional synthesis rate of hepatic nonsecretory protein. Both branched chain amino acid and L-alanine administration reduced endogenous tyrosine oxidation and improved N balance. Branched chain amino acid administration significantly increased amino acid incorporation into whole body protein and fractional synthetic rates of skeletal muscle, kidney and hepatic nonsecretory protein. The catabolic response to severe injury is consistent with increased rates of plasma amino acid appearance and a branched chain amino acid administration spares body protein by improving amino acid utilization for whole body protein synthesis.