an amino acid comprising about 1 to 6 percent by weight of the mixture obtained by hydrolysis of most proteins. First isolated from casein (1849), tyrosine is particularly abundant in insulin (a hormone) and papain (an enzyme found in fruit of the papaya), which contain 13 percent by weight. Tyrosine is one of several so-called essential amino acids for certain animals; i.e., they cannot synthesize it and require dietary sources. Other species can, however, convert phenylalanine, also an essential amino acid for fowl and mammals, to tyrosine whenever necessary for protein synthesis.