Phenylalanine: An essential amino acid. (The human body cannot make it so it is essential to the diet.) Phenylalanine that is ingested is largely transformed (hydroxylated) to form the amino acid tyrosine, which is used in protein synthesis. Too little phenylalanine curbs physical and intellectual growth. Too much phenylalanine, as in phenylketonuria (PKU), is highly toxic to the brain. Phenylanine was first isolated in 1879 and first synthesized in 1882. Symbol: Phe.