Table 1.

Describes the distinct properties of IDO/TDO in the Kyn pathway

	TDO	IDO	References
Substrate selectivity	L-Trp	L, D-Trp	[29, 30]
Distribution	Liver	Whole tissues	[29, 30]
Principle effector	Glucocorticoids	IFN-Y	[29, 30]
Subunite	Tetrameric (a2b2) (167 K)	Monomeric (41 K)	[29, 30]
Location	4q31	8p11.21	[29, 30]
Inhibitors	Many compounds (allopurinol, phenol,..)	Few compounds (mainly norharman)	[29, 30]
For catalytic activity uses	O2	superoxide anion (O−2)	[29, 30]