organic compound, one of the 20 amino acids commonly found in
animal proteins. Only the l -stereoisomer appears in mammalian protein. It is one of several essential amino acids needed in the diet; human beings cannot synthesize it from simpler metabolites. Young adults require about 7 mg of this amino
acid per day per kg (3 mg per lb) of body weight. Nicotinic
acid (niacin), a vitamin of the B complex, can be made from tryptophan in the body, but evidently the rate of transformation is insufficient for the demands of normal growth and maintenance, and hence nicotinic
acid must be supplied in the diet. Deficiency of tryptophan in the diet enhances the progress of the vitamin-deficiency disease pellagra , which is treated by restoring nicotinic
acid to the diet, usually supplemented with tryptophan. Bacteria in the intestine
break tryptophan down to compounds such as skatole and indole, which to a
great extent are responsible for the unpleasant odor of feces. Tryptophan contributes to the structure of proteins into which it has been incorporated by the tendency of its side chain to participate in hydrophobic interactions (see isoleucine ). The amino
acid was isolated from casein (milk protein) in 1901, and its structure was established in 1907.
