The enzyme reactions bringing about oxidation in the indole ring of tryptophan and kynurenine formation furnish a convenient example of those poorly understood systems producing aromatic ring oxidations. The metabolism of tryptophan via kynurenine has been studied in different instances by nutritional, isotopic, genetic, and adaptive experiments; thus this metabolic pathway has been well documented on animals (1, Z), bacteria (34, Neurospora (6), and insects (7, 8). The enzymes responsible for these reactions have received little attention. There are only the reports of Kotake and his school ascribing the reaction to a “tryptophan-pyrrolase” found in liver. The demonstration of this, by day long incubation under paraffin (9), yielded no definite reaction in our hands.

We have been able to demonstrate kynurenine formation from tryptophan in liver homogenates under aerobic conditions (10). In this paper the over-all reaction to kynurenine is described, and the first two steps are identified as a physiological, coupled oxidation system consisting of a tryptophan peroxidase and an oxidase which forms peroxide. Tryptophan oxidation by this system yields formylkynurenine. The identification of formylkynurenine and of the enzyme formylase which hydrolyzes it to formic acid and kynurenine is described in Paper II of this series (11).