Table 1 Tryptophan metabolites that can bind AHR and are expressed in CKD

Indoxyl sulfate [24]

(activates endothelial oxidative stress/endothelial dysfunction)

Elevated in serum, 23.1 mg/mL versus

0.53 mg/mL in healthy subjects [152]

Gut bacteria

Indole-3-acetic acid [123]

(inhibits endothelial cell angiogenesis)

Elevated in serum, 2 mg/mL versus

0.5 mg/mL in healthy subjects [152]

Gut bacteria

Indole 3-propionic acid [153] (activates macrophage phagocytosis)

Reduced in serum, 34.7 ng/mL versus

49.8 ng/mL in healthy subjects [154]

Gut bacteria

Kynurenic acid [155]

(induces epithelial IL-6 production, also an HCAR3 ligand)

Elevated in serum, 151.0 ug/L versus 5.48 ug/mL in healthy subjects [152]

Kynurenine aminotransferases

Kynurenine [156]

(activates T regulatory cells and immunosuppressive myeloid cells)

Elevated in plasma (uM), CKD stage 3 (2.61), CKD stage 4 (3.17), CKD stage 5 (3.72) versus health subjects (1.84)

[157]

IDO1