Table 1.

The respective roles of kynurenine and its downstream metabolites.

Compound	Blood-brain barrier crossing rate	Role in body tissues
KYN	fast	Direct neuroprotective effects (15). Detection of toxic substances in the body and reduction of the risk of diseases caused by them through activation of AhR (13). Activation of AhR exerts an immunosuppressive effect and promote the immune escape of tumor cells (13). Inhibition of osteoblast differentiation and promotion of osteoclast action (83). Enhance the differentiation of Treg cells, modulates immune response (12). Vasodilation (62).
QA	slow	Agonizes NMDA receptors leading to neurotoxicity (21). Promote oxidative stress (26). Alleviate skin inflammation through AhR (29).
KYNA	slow	Neuroprotective effect (37). Anti-inflammatory effect (21). Myocardial ischemic protective effect (22).
3-HAA	fast	Reduce local vascular inflammation and atherosclerosis (31, 97).
3-HK	fast	Neurotoxic, accumulates and damages nerve cells (23). Induced apoptosis of immune cells and temper inflammatory response (64).
KMO	slow	Catalyze neurotoxic compounds production (34). Exerts inflammatory modulation (38).

KYN, kynurenine; QA, quinolinic acid; NMDA, N-methyl-D-aspartate; AhR, aromatic hydrocarbon receptor; KYNA, kynurenic acid; 3-HAA, 3-hydroxyanthranilic acid; 3-HK, 3-hydroxykynurenine; KMO, kynurenine 3-monooxygenase.