Tryptophan Metabolism in Man *

The amino acid tryptophan is unique because it contains the indole nucleus and because it is metabolized in man through several different bio- chemical pathways to a number of specific prod- ucts. It is the precursor of serotonin and 5-hy- droxyindoleacetic acid. In addition, after cleavage of the indole ring, it may be metabolized by way of the kynurenine pathway to 3-hydroxyanthranilic acid and ultimately to nicotinamide. In mam- malian liver the benzene ring is oxidized and me- tabolized through a number of intermediate reac- tions to glutarate, acetate, and carbon dioxide (1, 2). Tryptophan is also the precursor of in- dolic acids, such as 3-indoleacetic acid. In man this compound is formed both by tissue enzymes and by bacteria in the gut (3). In the intestinal tract, bacteria that contain tryptophanase (4) re- ductively cleave the side chain of tryptophan and form indole, which is absorbed, conjugated in the liver, and excreted as indican (sulfated potassium ester of indoxyl). In addition to the various re- actions involving the indole ring or side chain, tryptophan, like other amino acids, is incorporated into protein. These pathways are schematically shown in Figure