Metabolism of l-Tryptophan to Kynurenate and Quinolinate in the Central Nervous System: Effects of 6-Chlorotryptophan and 4-Chloro-3-Hydroxyanthranilate

Abstract: The metabolism of l -tryptophan to the neuroactive kynurenine pathway metabolites, l -kynurenine, kynurenate and quinolinate, and the effects of two inhibitors of quinolinate synthesis (6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate) were investigated by mass spectrometric assays in cultured cells and in vivo. Cell lines obtained from astrocytoma, neuroblastoma, macrophage/monocytes, lung, and liver metabolized l -¹³C₆]-tryptophan to l -¹³C₆]kynurenine and ¹³C₆]kynurenate, particularly after indoleamine-2,3-dioxygenase induction by interferon-γ. Kynurenine aminotransferase activity was measurable in all cell types examined but was unaffected by interferon-γ. These results suggest that many cell types can be sources of kynurenate following immune activation. In vivo synthesis of l -¹³C₆]kynurenine and ¹³C₆]kynurenate from l -¹³C₆]tryptophan was studied in the CSF of macaques infected with poliovirus, as a model of inflammatory neurologic disease. The effects of 6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate on the synthesis of kynurenate were different. 6-Chlorotryptophan attenuated formation of l -¹³C₆]kynurenine and ¹³C₆]kynurenate and was converted to 4-chlorokynurenine and 7-chlorokynurenate. It may be an effective prodrug for the delivery of 7-chlorokynurenate, which is a potent antagonist of NMDA receptors. In contrast, 4-chloro-3-hydroxyanthranilate did not reduce accumulation of l -¹³C₆]kynurenine and ¹³C₆]kynurenate. 6-Chlorotryptophan and 4-chloro-3-hydroxyanthranilate are useful tools to manipulate concentrations of quinolinate and kynurenate in the animal models of neurologic disease to evaluate physiological roles of these neuroactive metabolites.