{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,3]],"date-time":"2026-08-03T18:00:08Z","timestamp":1785780008497,"version":"3.56.0"},"reference-count":117,"publisher":"MDPI AG","issue":"19","license":[{"start":{"date-parts":[[2022,10,2]],"date-time":"2022-10-02T00:00:00Z","timestamp":1664668800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"C2F program, Chulalongkorn University","award":["64.310\/169\/2564"],"award-info":[{"award-number":["64.310\/169\/2564"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Cells"],"abstract":"<jats:p>Major depressive disorder (MDD) and bipolar disorder (BD) with melancholia and psychotic features and suicidal behaviors are accompanied by activated immune-inflammatory and oxidative pathways, which may stimulate indoleamine 2,3-dioxygenase (IDO), the first and rate-limiting enzyme of the tryptophan catabolite (TRYCAT) pathway resulting in increased tryptophan degradation and elevated tryptophan catabolites (TRYCTAs). The purpose of the current study is to systematically review and meta-analyze levels of TRP, its competing amino acids (CAAs) and TRYCATs in patients with severe affective disorders. Methods: PubMed, Google Scholar and SciFinder were searched in the present study and we recruited 35 studies to examine 4647 participants including 2332 unipolar (MDD) and bipolar (BD) depressed patients and 2315 healthy controls. Severe patients showed significant lower (p &lt; 0.0001) TRP (standardized mean difference, SMD = \u22120.517, 95% confidence interval, CI: \u22120.735; \u22120.299) and TRP\/CAAs (SMD = \u22120.617, CI: \u22120.957; \u22120.277) levels with moderate effect sizes, while no significant difference in CAAs were found. Kynurenine (KYN) levels were unaltered in severe MDD\/BD phenotypes, while the KYN\/TRP ratio showed a significant increase only in patients with psychotic features (SMD = 0.224, CI: 0.012; 0.436). Quinolinic acid (QA) was significantly increased (SMD = 0.358, CI: 0.015; 0.701) and kynurenic acid (KA) significantly decreased (SMD = \u22120.260, CI: \u22120.487; \u22120.034) in severe MDD\/BD. Patients with affective disorders with melancholic and psychotic features and suicidal behaviors showed normal IDO enzyme activity but a lowered availability of plasma\/serum TRP to the brain, which is probably due to other processes such as low albumin levels.<\/jats:p>","DOI":"10.3390\/cells11193112","type":"journal-article","created":{"date-parts":[[2022,10,8]],"date-time":"2022-10-08T01:52:21Z","timestamp":1665193941000},"page":"3112","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":32,"title":["The Tryptophan Catabolite or Kynurenine Pathway in a Major Depressive Episode with Melancholia, Psychotic Features and Suicidal Behaviors: A Systematic Review and Meta-Analysis"],"prefix":"10.3390","volume":"11","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7667-6731","authenticated-orcid":false,"given":"Abbas F.","family":"Almulla","sequence":"first","affiliation":[{"name":"Department of Psychiatry, Faculty of Medicine, Chulalongkorn University, Bangkok 10330, Thailand"},{"name":"Medical Laboratory Technology Department, College of Medical Technology, The Islamic University, Najaf 31001, Iraq"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yanin","family":"Thipakorn","sequence":"additional","affiliation":[{"name":"Department of Psychiatry, Faculty of Medicine, Chulalongkorn University, Bangkok 10330, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Asara","family":"Vasupanrajit","sequence":"additional","affiliation":[{"name":"Department of Psychiatry, Faculty of Medicine, Chulalongkorn University, Bangkok 10330, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Chavit","family":"Tunvirachaisakul","sequence":"additional","affiliation":[{"name":"Department of Psychiatry, Faculty of Medicine, Chulalongkorn University, Bangkok 10330, Thailand"},{"name":"Cognitive Impairment and Dementia Research Unit, Faculty of Medicine, Chulalongkorn University, Bangkok 10330, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Gregory","family":"Oxenkrug","sequence":"additional","affiliation":[{"name":"Department of Psychiatry, Tufts University School of Medicine and Tufts Medical Center, Boston, MA 02111, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6143-5196","authenticated-orcid":false,"given":"Hussein K.","family":"Al-Hakeim","sequence":"additional","affiliation":[{"name":"Department of Chemistry, College of Science, University of Kufa, Kufa 54002, Iraq"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Michael","family":"Maes","sequence":"additional","affiliation":[{"name":"Department of Psychiatry, Faculty of Medicine, Chulalongkorn University, Bangkok 10330, Thailand"},{"name":"Department of Psychiatry, Medical University of Plovdiv, 4002 Plovdiv, Bulgaria"},{"name":"School of Medicine, Barwon Health, IMPACT, The Institute for Mental and Physical Health and Clinical Translation, Deakin University, Geelong, VIC 3217, Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2022,10,2]]},"reference":[{"key":"ref_1","unstructured":"American Psychiatric Association (2000). 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