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Positive identification of kynurenine in rat and human brain [proceedings].

The major degradative pathway in mammals for the essential amino acid tryptophan is initiated by oxidative cleavage of the pyrrole ring. The first readily detectable metabolite on this pathway is kynurenine. Its formation is measured in the assay in uitro of the liver enzyme catalysing this reaction, tryptophan pyrrolase (tryptophan 2,3-dioxygenase, EC 1.13.11.11), normally by monitoring A365 (Knox & Auerbach, 1955; Feigelson & Greengard, 1961). Kynurenine determination in physiological fluids, sometimes after a tryptophan load, has been used as an index of tryptophan pyrrolase activity in vioo in man and animals (Price et al., 1965; Joseph et al., 1976). Limitations in the sensitivity and selectivity of current assays for kynurenine, and hence for the assay of tryptophan pyrrolase, prompted the development of a sensitive g.1.c. method for kynurenine (Naruse et af., 1973; Joseph 1977). Indoleamine 2,3dioxygenase (Hayaishi, 1976) catalyses the same reaction as tryptophan pyrrolase, but has a broader substrate specificity. It has been described in intestine and in a number of other tissues, including brain (Gal, 1974; Hayaishi, 1976). The present communication describes the application of the new kynurenine assay to brain tissue. Brain tissue was homogenized in 5vol. of acid butanol and back-extracted into 1 vol. of 0 .1~-HCl as described by Curzon & Green (1970). The 0.1 M-HCl phase was made alkaline with one-third of its volume of 1OM-NaOH, and Tiron (1,2-dihydroxybenzene-