Human Endogenous Natural Products
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N
H
O
NH2
O
O H
N-formylkynurenine
NH2
O
NH2
O
OH
kynurenine
NH2
O
OH
anthranilic acid
N
H
indole-3-aldehyde
N
H
O
OH
indole-3-acrylic acid
N
H
OSO3H
3-indoxyl sulfate
N
H
O
OH
indole-3-carboxylic acid
N
H
O
O
OH
indole-3-pyruvate
N
H
O
OH
indole-3-acetic acid
H
N
indole
N
H
OH
O
OH
indole-3-lactic acid
N
H
O
OH
indole-3-propionic acid
N
H
N
N,N-dimethyltryptamine
N
H
O
NH2
indole-3-acetamide
HN
H
N
O
N-acetyltryptamine
N
H
3-methylindole
NH2
H
N
tryptamine
OH
H
N
tryptophol
NH2
O
NH2
O
OH
HO
3-hydroxykynurenine
NH2
H
N
O
5-methoxytryptamine
NH2 O
OH
HO
3-hydroxyanthranilic acid
OH
OH
N
O
OH
xanthurenic acid
HN
O
H
N
O
melatonin
N
H
O
OH
HO
5-hydroxyindole-3-acetic acid
NH2
H
N
O
OH
HO
5-hydroxytryptophan
NH2
H
N
HO
serotonin
HN
H
N
HO
N-methylserotonin
HN
H
N
HO
O
N-acetylserotonin
NH2
H
N
O
OH
F
F-tryptophan
kynurenine pathway metabolites
serotonin pathway metabolites
commensal bacterial metabolites
O
O
kynurenic acid
N
OH
OH
O
Fig. 8 Diverse tryptophan metabolites in the gut that are produced by different pathways. The
compounds marked in blue are produced by two liver pathways including the kynurenine pathway
and the serotonin pathway, while the compounds marked in red come from the feces. The circled
compound, 6-F-tryptophan, is used as a labeled tryptophan to study the origin of indole-related
metabolites [16]