Table 2.

Expected changes in plasma tryptophan and kynurenine metabolites after up- or downregulation (inhibition) of kynurenine pathway enzymes.

Enzyme	Trp	Kyn	[Kyn]/[Trp]	KA	AA	3-HK	XA	3-HAA	QA
TDO↑	↓	↑	↑	↑	↑	↑	↑	↑	↑
↓	↑	↓	↓	↓	↓	↓	↓	↓	↓
IDO↑	↓?	↑	↑	↑	↑	↑	↑	↑	↑
↓	−	↓	↓	↑	↑	↑	↑	↑	↑
KMO↑	−	↓	↓	↓	↓	↑	↑	↑	↑
↓	−	↑	↑	↑	↑	↓	↓	↓	↓
KAT↑	−	(↑)	↑	↑	↑	−	↑	−	−
↓	−	?	?	↓	?	?	↓	?	?
KYNU↑	−	−	−	−	↑	−	−	↑	↑
↓	−	↑	↑	↑?	↓	↑	↑	↓	↓

Abbreviations: AA, anthranilic acid; 3-HAA, 3-hydroxyanthranilic acid; 3-HK, 3-hydroxykynurenine; KA, kynurenic acid; IDO, indoleamine 2,3-dioxygenase; KAT, kynurenine aminotransferase; KMO, kynurenine monooxygenase; KYNU, kynureninase; QA, quinolinic acid; TDO, tryptophan 2,3-dioxygenase; Trp, tryptophan; XA, xanthurenic acid.