Abnormalities of Phenylalanine and Tyrosine Kinetics: Significance in Septic and Nonseptic Burned Patients

The flow of phenylalanine, the essential amino acid precursor of thyroid hormone and catecholamines, was severely elevated in five septic burned patients (6.70 ± 1.07 mg/kg) and six nonseptic burned patients (5.00 ± 0.44 mg/kg) when compared with seven normal controls (2.10 ± 0.33 mg/kg). Fasting serum concentrations of phenylalanine were elevated in the septic burned patients (2.33 ± 0.37 mg/100 ml of serum) relative to the nonseptic patients (1.28 ± 0.21 mg/100 ml) and the controls (1.01 ± 0.15 mg/100 ml). The rate of appearance of the phenylalanine metabolite, tyrosine, after an oral phenylalanine dose was normal in all burn patients. Increased serum concentrations and increased flow of phenylalanine are an index of rapid protein catabolism, further augmented by sepsis in the thermally injured, and not a reflection of decreased hepatic conversion of phenylalanine to tyrosine.