tyrosine (tĪˈrəsēn) [key], organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l -stereoisomer appears in mammalian protein. It is not essential to the human diet, since it can be synthesized in the body from phenylalanine. When the enzyme that catalyzes the transformation of phenylalanine to tyrosine is not active because of a hereditary defect, the serious disease known as phenylketonuria (PKU) results. Other defects in tyrosine metabolism include the rare hereditary disorder known as alkaptonuria, characterized by discharge of a urine which darkens on standing exposed to air. Tyrosine is a precursor of the adrenal hormones epinephrine and norepinephrine as well as of the thyroid hormones, including thyroxine. Melanin, the skin and hair pigment, is also derived from this amino acid. Tyrosine residues in enzymes have frequently been shown to be associated with active sites. Modification of these residues with various chemicals often results in a change in the specificity of the enzyme toward its substrates or even in total destruction of its activity. In 1846 tyrosine was obtained as a product of the degradation of the protein casein (from cheese). It was synthesized in the laboratory in 1883, and its structure was thus determined.