A new metabolic pathway of tryptophan initiated by tryptophan side chain oxidase.

A new metabolic pathway of L-tryptophan, tentatively referred to as the side chain pathway, was demonstrated in Pseudomonas fluorescens (ATCC 29574), in which the initial reaction was catalyzed by tryptophan side chain oxidase (Takai, K., Ushiro, H., Noda, Y., Narumiya, S., Tokuyama, T., and Hayaishi, 0. (1977) J. Biol. Chem. 252,2648-2656). When the bacteria harvested in the late stationary phase of growth were incubated with L-tryptophan, approximately 2 mol of 02 were consumed per mol of Ltryptophan. By thin layer chromatography and gas chromatography/mass spectrometry, three indole acids (3-indoleacetic acid, 3Gndoleglycolic acid, and 3indoleglyoxylic acid) and the corresponding three indole alcohols (3Gndoleethano1, 3-indoleglycol, and 3indoleketol) were identified as metabolites in addition to kynurenine, anthranilic acid, and kynurenic acid. Although not accumulated in significant quantities, the occurrence of 3Gndoleacetaldehyde and 3-indoleglycolaldehyde/3-indoleglyoxal was shown by the isotope trapping experiment and by the formation of 2-(3-indolyl)quinoxaline in the presence of o-phenylenediamine. Upon incubation with the cells, these indole aldehydes were converted to the corresponding indole acids and alcohols described above. On the basis of these results together with those obtained in the cell-free system, a new metabolic pathway is proposed in which L-tryptophan is metabolized to 3Gndoleacetaldehyde or 3-indoleglycolaldehyde and then to 3Gndoleglyoxal by the action of tryptophan side chain oxidase, and each of these indole aldehydes undergoes further oxidation and reduction to the corresponding indole acid and alcohol by the action of aldehyde dehydrogenase and alcohol dehydrogenase, respectively. 3-Indoleacetic acid and 3-indoleethanol are then oxidized to 3Gndoleglyoxylic acid and 3-indoleketol via 3-indoleglycolic acid and 3-indoleglycol, respectively, by the action of tryptophan side chain oxidase. This pathway was induced immediately after the cessation of cell growth and fully active during the sta-