N-Acetyl-L-phenylalanine (N-Ac-Phenylalanine) is an essential amino acid produced for medical, feed, and nutritional applications. It appears in large amount in urine of patients with phenylketonuria which is a human genetic disorder due to the lack of phenylalanine hydroxylase, the enzyme necessary to metabolize phenylalanine to tyrosine. Acetylphenylalanine is a product of enzyme phenylalanine N-acetyltransferase in the pathway phenylalanine metabolism.