The Kynurenine-Anthranilic Acid Pathway

The initial degradation products by this pathway are formylkynurenine, ksmurenine, 3-hydroxykynurenine, anthranilic acid or 3-hydroxyanthranilic acid, and alanine. 

The first of the enzymes catalyzing this sequence of reactions is now named tryptophan pyrrolase. The reaction promoted is given in Eq. (19). 

Formation of kynurenine from tryptophan was discovered in liver extracts by Kotake and Masayama 289). These authors proposed the name tryptophan pyrrolase for the enzyme. Knox and Mehler 290) subsequently showed that the formation of kynurenine consisted of two enzyme reactions, an initial oxidation to formylkynurenine followed by hydrolysis to kynurenine. Because the reaction was stimulated by HsO produced in situ it was assumed that there was an intermediate formation and utilization of peroxide in the oxidation. For this reason the enzyme was renamed tryptophan oxidase-peroxidase by Knox. Experiments with O showed that molecular O2 was incorporated into the reaction products 291). One mole of O2 per mole of tryptophan was contained in the formylkynurenine. When H20 was tested, very little 0 was utilized. This observation led Tanaka and Knox 292) to return to the use of the original name, tryptophan pyrrolase. 

Other properties of tryptophan pyrrolase that have been established in the course of time are (a) one molecule of oxygen is utilized in a single step b) the enzyme is inhibited by catalase and regenerated by H2O2 under appropriate conditions (c) it is inhibited by cyanide, hydroxylamine, and sodium azide d) it is inhibited by CO in the dark and this is reversed by light. 

These results suggested that the enzyme was an iron porphyrin protein and this has been verified with a bacterial preparation from Pseudomonas fluorescens 292). 

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