Kynurenine is synthesized by the enzyme tryptophan dioxygenase, which is made primarily but not exclusively in the liver, and indoleamine 2,3-dioxygenase, which is made in many tissues in response to immune activation.[1] Kynurenine and its further breakdown products carry out diverse biological functions, including dilating blood vessels during inflammation[2] and regulating the immune response.[3] Some cancers increase kynurenine production, which increases tumor growth.[1]
Kynurenine protects the eye by absorbing UV light, especially in the UVA region (315-400 nm).[4] Kynurenine is present in the lens and retina as one of multiple tryptophan derivatives produced in the eye, including 3-hydroxykynurenine, that together provide UV protection and aid in enhancing visual acuity.[5][6] The use of kynurenine as a UV filter is consistent with its photostability and low photosensitization, owing to its efficient relaxation from the UV-induced excited state.[7] The concentration of this UV filter decreases with age,[8] and this loss of free kynurenine and the concomitant formation of relatively more photosensitizing kynurenine derivatives and kynurenine-protein conjugates may contribute to the formation of cataracts.[9][10][11]
Evidence suggests that increased kynurenine production may precipitate depressive symptoms associated with interferon treatment for hepatitis C.[12] Cognitive deficits in schizophrenia are associated with imbalances in the enzymes that break down kynurenine.[13] Blood levels of kynurenine are reduced in people with bipolar disorder.[14] Kynurenine production is increased in Alzheimer's disease[15][16] and cardiovascular disease[17] where its metabolites are associated with cognitive deficits[18] and depressive symptoms.[19] Kynurenine is also associated with tics.[20][21]
^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. 26 (7): 3419–3429. doi:10.1038/s41380-020-00913-1. PMID33077852. S2CID224314102.
^Guillemin GJ, Brew BJ, Noonan CE, Takikawa O, Cullen KM (2005). "Indoleamine 2,3 dioxygenase and quinolinic acid Immunoreactivity in Alzheimer's disease hippocampus". Neuropathology and Applied Neurobiology. 31 (4): 395–404. doi:10.1111/j.1365-2990.2005.00655.x. PMID16008823. S2CID7754894.
^Wirleitner B, Rudzite V, Neurauter G, Murr C, Kalnins U, Erglis A, Trusinskis K, Fuchs D (2003). "Immune activation and degradation of tryptophan in coronary heart disease". European Journal of Clinical Investigation. 33 (7): 550–4. doi:10.1046/j.1365-2362.2003.01186.x. PMID12814390. S2CID10300941.
^Gulaj E, Pawlak K, Bien B, Pawlak D (2010). "Kynurenine and its metabolites in Alzheimer's disease patients". Advances in Medical Sciences. 55 (2): 204–11. doi:10.2478/v10039-010-0023-6. PMID20639188.
^Swardfager W, Herrmann N, Dowlati Y, Oh PI, Kiss A, Walker SE, Lanctôt KL (2009). "Indoleamine 2,3-dioxygenase activation and depressive symptoms in patients with coronary artery disease". Psychoneuroendocrinology. 34 (10): 1560–6. doi:10.1016/j.psyneuen.2009.05.019. PMID19540675. S2CID36687413.
^Hoekstra PJ, Anderson GM, Troost PW, Kallenberg CG, Minderaa RB (2007). "Plasma kynurenine and related measures in tic disorder patients". European Child & Adolescent Psychiatry. 16: 71–7. doi:10.1007/s00787-007-1009-1. PMID17665285. S2CID39150343.