an amino acid present in the mixture obtained upon hydrolysis of common proteins. Human hemoglobin (the oxygen-carrying pigment of red blood cells) is one of the richest sources of phenylalanine, yielding 9.6 percent by weight. First isolated in 1881 from lupine seedlings, phenylalanine is one of several essential amino acids for fowls and mammals; i.e., they cannot synthesize it and require dietary sources. Microorganisms synthesize it from glucose and pyruvic acid (products of the breakdown of carbohydrates).