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ABSTRACT. Background: N-acetyl-L-tyrosine (NAT) is commonly used in place of tyrosine in parenteral nutrition, but human studies carried out to date indicate considerable amounts of it are excreted unchanged in the urine. NAT retention has not been well studied in parenterally fed adults. Methods: NAT retention was measured in 13 adults receiving continuous parenteral nutrition with Aminosyn II 15% (Abbott Laboratories, Abbott Park, IL). Results: Approximately 35% of administered NAT was excreted unchanged in the urine, with no important effect of infusion rate, N balance, or level of renal function on this value. Sufficient NAT was retained that the prescription of 1 g total amino acids/kg-day^sup -1^ using this product exceeded the combined recommended dietary allowance for aromatic amino acids Conclusion: As used in the clinical setting, the phenylalanine and NAT composition of Aminosyn II is sufficient to meet the combined aromatic amino acid needs of adults with normal phenylalanine hydroxylase activity. (Journal of Parenteral and Enteral Nutrition 27:419-422, 2003)
Safe and effective delivery of parenteral nutrition requires the infusion of crystalline amino acids in physiologic amounts and proportions.1 Tyrosine's limited water solubility poses a problem in this regard, and for this reason, the 10% formulation of the widely used product, Aminosyn (Abbott Laboratories, SaintLaurent, QC) contains only 44 mg tyrosine/100 mL. Tyrosine is synthesized from phenylalanine in the liver and kidney,2 so when phenylalanine hydroxylase activity is normal, a sufficiently large amount of phenylalanine will compensate for absent tyrosine in a nutrient admixture. Recently published recommended dietary allowances (RDA) for phenylalanine and tyrosine are expressed as their sum: 36 mg/kg-day^sup -1^.3 Aminosyn 10% provides 440 mg phenylalanine/100 mL, so a patient infused 1.0 g/kg-day^sup -1^ total amino acids from this product will receive 44 mg phenylalanine/kg-day^sup -1^ and 4 mg tyrosine/kg-day^sup -1^. This exceeds the combined aromatic amino acid RDA and an earlier estimate that the average normal phenylalanine requirement of adults, in the absence of tyrosine, is 39 mg/kg-day^sup -1^.4
The water solubility of N-acetyl-L-tyrosine (NAT) initially suggested it could be used as a substitute for tyrosine in amino acid mixtures, and this was justified by rat studies demonstrating its prompt deacylation to tyrosine and incorporation into proteins.5 The situation is more complicated in humans, however.6 Heird et al7,8 measured plasma amino...