Studies on alterations of free aromatic amino acids and protein turnover in skeletal muscles under surgical stresses.

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We examined the relationship between the elevation of blood phenylalanine concentrations often observed In trauma or infected patients without hepatic dysfunction and alterations of liver phenylalanine catabolism in rats undergoing pathophysiologically different stresses, either sepsis or scald injury. The adenine nucleotide levels and the energy state in tissues of abodominal and femoral muscles and diaphragm were maintained by breakdown of phosphocreatine, but protein degradation was promoted in these tissues. The alterations of many amino acid levels in abdominal and femoral muscles paralleled those of the corresponding amino acid levels in plasma under these stresses. The observed changes in free amino acids in plasma and the muscles well reflected the enhanced protein degradation in skeletal muscles. Phenylalanine hydroxylase activity, as measured using a naturally occurring cofactor, decreased by 40%, while phenylalanine-pyruvate aminotransferase was activated by 25% in the livers of the septic rats as compared with those of the controls. Such activation, caused by an increase in glucagon levels, resulted in a 76% increase in oxidation of phenylalanine over that of controls, as measured by pulse-administration of ^<14>C-labeled phenylalanine. In the scalded rats whose plasma phenylalanine level showed a comparable but temporary rise, no significant alterations in these enzyme activities and phenylalanine oxidation occurred. These results indicate that reduction of phenylalanine hydroxylation was also responsible for the elevation of the plasma phenylalanine level in infected rats during massive mobilization of amino acids from skeletal muscles under conditions of enhanced and sustaining catabolism.