Chemistry:Kynurenic acid

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Kynurenic acid
Chemical structure of kynurenic acid
Ball-and-stick model of kynurenic acid
IUPAC name
4-hydroxyquinoline-2-carboxylic acid
Other names
Kinurenic acid, kynuronic acid, quinurenic acid, transtorine
3D model (JSmol)
Molar mass 189.168 g/mol
Melting point 282.5 °C (540.5 °F; 555.6 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Kynurenic acid (KYNA or KYN) is a product of the normal metabolism of amino acid L-tryptophan. It has been shown that kynurenic acid possesses neuroactive activity. It acts as an antiexcitotoxic and anticonvulsant, most likely through acting as an antagonist at excitatory amino acid receptors. Because of this activity, it may influence important neurophysiological and neuropathological processes. As a result, kynurenic acid has been considered for use in therapy in certain neurobiological disorders. Conversely, increased levels of kynurenic acid have also been linked to certain pathological conditions.

Kynurenic acid was discovered in 1853 by the German chemist Justus von Liebig in dog urine, which it was apparently named after.[1]

It is formed from L-kynurenine in a reaction catalyzed by the enzyme kynurenine—oxoglutarate transaminase.

Mechanism of action

KYNA has been proposed to act on five targets:

  • As an antagonist at ionotropic AMPA, NMDA and Kainate glutamate receptors in the concentration range of 0.1-2.5 mM.[2]
  • As a noncompetitive antagonist at the glycine site of the NMDA receptor.
  • As an antagonist of the α7 nicotinic acetylcholine receptor.[3] However, recently (2011) direct recording of α7 nicotinic acetylcholine receptor currents in adult (noncultured) hippocampal interneurons by the Cooper laboratory [4] validated a 2009 study[5] that failed to find any blocking effect of kynurenic acid across a wide range of concentrations, thus suggesting that in noncultured, intact preparations from adult animals there is no effect of kynurenic acid on α7 nicotinic acetylcholine receptor currents.[4][5]
  • As a ligand for the orphan G protein-coupled receptor GPR35.[6]
  • As an agonist for the G protein-coupled receptor HCAR3.[7]

Role in disease

High levels of kynurenic acid have been identified in patients suffering from tick-borne encephalitis,[8] schizophrenia and HIV-related illnesses. In all these situations increased levels were associated with confusion and psychotic symptoms. Kynurenic acid acts in the brain as a glycine-site NMDAr antagonist, key in glutamatergic neurotransmission system, which is thought to be involved in the pathophysiology and pathogenesis of schizophrenia.

A kynurenic acid hypothesis of schizophrenia was proposed in 2007,[9][10] based on its action on midbrain dopamine activity and NMDArs, thus linking dopamine hypothesis of schizophrenia with the glutamate hypothesis of the disease.

Kynurenic acid is reduced in individuals suffering from bipolar disorder, especially during depressive episodes.[11]

High levels of kynurenic acid have been identified in human urine in certain metabolic disorders, such as marked pyridoxine deficiency and deficiency/absence of kynureninase.

When researchers decreased the levels of kynurenic acid in the brains of mice, their cognition was shown to improve markedly.[12]

Kynurenic acid shows neuroprotective properties.[13] Some researchers have posited that the increased levels found in cases of neurological degradation is due to a failed attempt to protect the cells.[14]

Link to ketogenic diet

One controlled study kept mice on a ketogenic diet and measured kynurenic acid concentrations in different parts of the brain.[15] It found that the mice on the ketogenic diet had greater kynurenic acid concentrations in the striatum and hippocampus compared to mice on a normal diet, with no significant difference in the cortex.

In response to the studies showing detrimental behaviour following increases in kynurenic acid[16] the authors also note that the diet was generally well tolerated by the animals, with no "gross behavioural abnormalities". They posit that the increases in concentrations found were insufficient to produce behavioural changes seen in those studies.

See also


  1. Liebig, J., Uber Kynurensäure, Justus Liebigs Ann. Chem., 86: 125-126, 1853.
  2. Elmslie, KS; Yoshikami, D (1985). "Effects of kynurenate on root potentials evoked by synaptic activity and amino acids in the frog spinal cord". Brain Res. 330 (2): 265–72. doi:10.1016/0006-8993(85)90685-7. 
  3. Hilmas, C.; Pereira, EFR; Alkondon, M.; Rassoulpour, A.; Schwarcz, R.; Albuquerque, E.X. (2001). "The Brain Metabolite Kynurenic Acid Inhibits α7 Nicotinic Receptor Activity and Increases Non-α7 Nicotinic Receptor Expression: Physiopathological Implications". J. Neurosci. 21 (19): 7463–7473. doi:10.1523/JNEUROSCI.21-19-07463.2001. 
  4. 4.0 4.1 Dobelis, P.; Varnell, A.; Cooper, Donald C. (2011). "Nicotinic α7 acetylcholine receptor-mediated currents are not modulated by the tryptophan metabolite kynurenic acid in adult hippocampal interneurons". Nature Precedings. doi:10.1038/npre.2011.6277.1. 
  5. 5.0 5.1 Mok, MH; Fricker, AC; Weil, A; Kew, JN (2009). "Electrophysiological characterisation of the actions of kynurenic acid at ligand-gated ion channels". Neuropharmacology 57 (3): 242–249. doi:10.1016/j.neuropharm.2009.06.003. PMID 19523966. 
  6. "Kynurenic acid as a ligand for orphan G protein-coupled receptor GPR35". J. Biol. Chem. 281 (31): 22021–8. 2006. doi:10.1074/jbc.M603503200. PMID 16754668. 
  7. Kapolka, NJ; Taghon, GJ; Rowe, JB; Morgan, WM; Enten, JF; Lambert, NA; Isom, DG (9 June 2020). "DCyFIR: a high-throughput CRISPR platform for multiplexed G protein-coupled receptor profiling and ligand discovery.". Proceedings of the National Academy of Sciences of the United States of America 117 (23): 13117-13126. doi:10.1073/pnas.2000430117. PMID 32434907. 
  8. "Elevated cerebrospinal fluid kynurenic acid levels in patients with tick-borne encephalitis". J. Intern. Med. 272 (4): 394–401. 2012. doi:10.1111/j.1365-2796.2012.02539.x. PMID 22443218. 
  9. "The kynurenic acid hypothesis of schizophrenia". Physiol. Behav. 92 (1): 203–209. 2007. doi:10.1016/j.physbeh.2007.05.025. PMID 17573079. 
  10. Kynurenic acid and schizophrenia. Advances in Experimental Medicine and Biology. 527. 2003. 155–65. doi:10.1007/978-1-4615-0135-0_18. ISBN 978-1-4613-4939-6. 
  11. Bartoli, F; Misiak, B; Callovini, T; Cavaleri, D; Cioni, RM; Crocamo, C; Savitz, JB; Carrà, G (19 October 2020). "The kynurenine pathway in bipolar disorder: a meta-analysis on the peripheral blood levels of tryptophan and related metabolites.". Molecular Psychiatry. doi:10.1038/s41380-020-00913-1. PMID 33077852. 
  12. Robert Schwarcz; Elmer, Greg I; Bergeron, Richard; Albuquerque, Edson X; Guidetti, Paolo; Wu, Hui-Qiu; Schwarcz, Robert (2010). "Reduction of Endogenous Kynurenic Acid Formation Enhances Extracellular Glutamate, Hippocampal Plasticity, and Cognitive Behavior". Neuropsychopharmacology 35 (8): 1734–1742. doi:10.1038/npp.2010.39. PMID 20336058. 
  13. Urbańska, Ewa M.; Chmiel-Perzyńska, Iwona; Perzyński, Adam; Derkacz, Marek; Owe-Larsson, Björn (2014). "Endogenous Kynurenic Acid and Neurotoxicity". Handbook of Neurotoxicity. pp. 421–453. doi:10.1007/978-1-4614-5836-4_92. ISBN 978-1-4614-5835-7. 
  14. Zádori, D.; Klivényi, P.; Vámos, E.; Fülöp, F.; Toldi, J.; Vécsei, L. (2009). "Kynurenines in chronic neurodegenerative disorders: future therapeutic strategies". Journal of Neural Transmission 116 (11): 1403–1409. doi:10.1007/s00702-009-0263-4. ISSN 0300-9564. PMID 19618107. 
  15. Żarnowski, Tomasz; Chorągiewicz, Tomasz; Tulidowicz-Bielak, Maria; Thaler, Sebastian; Rejdak, Robert; Żarnowska, Iwona; Turski, Waldemar Andrzej; Gasior, Maciej (2011). "Ketogenic diet increases concentrations of kynurenic acid in discrete brain structures of young and adult rats". Journal of Neural Transmission 119 (6): 679–684. doi:10.1007/s00702-011-0750-2. ISSN 0300-9564. PMID 22200857. 
  16. Potter, Michelle C; Elmer, Greg I; Bergeron, Richard; Albuquerque, Edson X; Guidetti, Paolo; Wu, Hui-Qiu; Schwarcz, Robert (2010). "Reduction of Endogenous Kynurenic Acid Formation Enhances Extracellular Glutamate, Hippocampal Plasticity, and Cognitive Behavior". Neuropsychopharmacology 35 (8): 1734–1742. doi:10.1038/npp.2010.39. ISSN 0893-133X. PMID 20336058. 

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