IDO1 and major depressive disorder: IDO1 catalyzes L‐tryptophan catabolism to kynurenine and consequently limits the availability of L‐tryptophan for the generation of neurotransmitters like serotonin and melatonin.[43] N‐acetylserotonin directly binds to IDO1 and enhances IDO1 activity in vitro and in vivo.[44] Ultimately, RagA overexpression reduced several key depression‐related neurotransmitters and metabolites in transgenic mice.