Journal of Neurorhenii.strv I .ippincoit—Ravcn Publishers, Philadelphia © 1996 International Society for Neurochemistry Kynurenine Disposition in Blood and Brain of Mice: Effects of Selective Inhibitors of Kynurenine Hydroxylase and of Kynureninase Alberto Chiarugi, Raffaella Carpenedo, and Flavio Moroni Department o~ /‘reclinical and Clinical Pharmacology, University of Florence, Florence, Italy Abstract: To study the regulation of the synthesis of quin- olinic and kynurenic acids in vivo, we evaluated (a) the metabolism of administered kynurenine by measuring the content of its main metabolites 3-hydroxykynurenine, an- thranilic acid, and 3-hydroxyanthranilic acid in blood and brain of mice; (b) the effects of (m-nitrobenzoyl)alanine, a selective inhibitor of kynurenine hydroxylase and of (o- methoxybenzoyl)alanine, a selective inhibitor of kynureni- nase, on this metabolism; and (c) the effects of (a-me- thoxybenzoyl)alanine on liver kynureninase and 3-hydro- xykynureninase activity. The conclusions drawn from these experiments are (a) the disposition of administered kynurenine preferentially occurs through hydroxylation in brain and through hydrolysis in peripheral tissues; (b) (m- nitrobenzoyl)alanine, the inhibitor of kynurenine hydroxy- lase, causes the expected changes in brain kynurenine metabolism, such as a decrease of 3-hydroxykynurenine, and an increase of kynurenic acid; and (c) (o-methoxy- benzoyl) alanine, the kynureninase inhibitor, increases brain concentration of the cytotoxic compound 3-hydra- xykynurenine, and unexpectedly does not reduce brain concentration of 3-hydroxyanthranilic acid, the direct pre- cursor of quinolinic acid. Taken together, the experiments suggest that the systemic administration of a kynurenine hydroxylase inhibitor is a rational approach to increase the brain content of kynurenate and to decrease that of cytotoxic kynurenine metabolites, such as 3-hydroxyky- nurenine and quinolinic acid. Key Words: Quinolinic acid— Kynurenic acid— NMDA— Kynurenine— Kynuren- ne hydroxylase— Kynureninase. J. Neurochem. 67, 692—698 (1996). The opening of the indole ring of tryptophan occurs as a consequence of the action of the enzymes trypto- phan 2,3-dioxygenase (EC 1.13.1.2), which is located mainly in the liver but is also present in brain (Haber et al., 1993), and indoleamine 2,3-dioxygenase (EC 1.13.11.17), which has been identified in a number of extrahepatic tissues, including brain (Yoshida and Hayashi, 1987). This opening is the rate-limiting step pathway for the formation of a group of compounds collectively named “kynurenines” (Fig. I). At least two kynurenines, kynurenic acid (KYNA) and quino- linic acid (QUIN), are able to interact with the excit- atory amino acid receptors (Stone and Perkins, 1981: Perkins and Stone, 1982). KYNA is a noncompetitlve antagonist of the N-rnethyl-D-aspartate (NMDA) re- ceptor ion channel complex, acting on the strychnine- insensitive glycinc recognition site (Johnson and Ascher, 1987) with an 1C 5() value in the low micromo- lar range (Kessler et al., 1989; Pellegrini-Giampictro et al., 1989). At higher concentrations, it is also able to interact, as a competitive antagonist, on AMPA and kainate receptors (Stone, 1993). On the contrary. QUIN exerts opposite effects, acting as an NMDA receptor agonist able to cause excitotoxic lesions (Schwarcz et al., 1983). Another kynurenine metabo- lite, picolinic acid, has been shown recently to he an activator of macrophage function (Varesio et al., 1990) and, finally, 3-hydroxykynurenine (30H-.kynurenine) has been shown to possess cytotoxic effects in a neu- ronal cell line (Eastman and Guilarte, 1989). Excessive stimulation of the ionotropic glutamate receptors has been observed in a number of neurologi- cal diseases such as epilepsy, stroke, and several de- generative disorders (Meldrum and Garthwaite, 1990: Rogawski, 1992). Furthermore, changes in the concen- tration of toxic kynurenines have been demonstrated in the course of infections involving the central nervous system (Heyes et al., 1991 ) and in models of multiple sclerosis (Flanagan et al., 1995). In view of the possi- ble importance of kynurenine metabolites in such a large variety of central nervous system pathologies, we previously investigated the effects of several inhibi- tors of the enzymes involved in kynurenine metabolism in rodents. Such inhibitors were tested in an attenipt Received November 6, 1995; revised manuscript received March 12, 1996; accepted March 19, 1996. Address correspondence and reprint requests to Dr. F. Moroni al Dipartimento di Farmacologia Preclinica e Clinica, Università di Firenze, Viale Morgagni 65, 50124 Firenze, Italy. Abbreviations used: KYNA, kynurenic acid; NMDA, N-methyl- D-aspartate; mNBA, (mn-nitrohenzoyl) alaninc; oMBA, (o-rnethoxy- benzoyl )alanine; 30H-kynurenine, 3-hydroxykynurenine; 30H-an- thranilic acid, 3-hydroxyanthranilic acid; QUIN, quinolinic acid. 692