tryptophan, anamino acid that is nutritionally important and occurs in small amounts inproteins. It is an essential amino acid, meaning that humans and certain other animals cannotsynthesize it and must obtain it from their diets. Infants require greater amounts of tryptophan than adults to ensure normal growth and development. Tryptophan is used by the body to manufacture several important substances, including theneurotransmitterserotonin and thevitaminniacin. Diets poor in tryptophan can lead topellagra, a disease resulting from niacin deficiency; however, this disease is now rare in developed countries. In 1901 the English biochemistFrederick G. Hopkins isolated tryptophan fromcasein, the majorprotein found inmilk. The chemical structure of tryptophan is