Figure 1.

IDO pathway and inhibitors.

Notes: Tryptophan catabolism through the kynurenine pathway involves IDO an intracellular rate limiting enzyme and possible natural and synthetic inhibitors of this enzyme acting through various routes. IDO serves as an apoenzyme which is encoded by the Indo gene, this gene is upregulated by JAK/STAT signaling pathway induced by IFN-γ, cyclooxygenase-mediating prostaglandin E2, TNF-α and LPS regulated by NF-κB. In contrast, it is downregulated by BIN gene, IL-4, nitric oxide and TGF-β through mRNA degradation pathway. Apart from the natural and synthetic inhibitors certain tryptophan active products, such as tryptamine and DMT, also serve as modulators of this enzyme.

Abbreviations: COX-2, cyclooxygenase-2; DMT, N-dimethyltryptamine; IDO, indoleamine 2,3-dioxygenase; IFN- γ, interferon gamma; IL, interleukin; JAK, Janus kinase; LPS, lipopolysaccharides; mRNA, messenger RNA; NAD, aldehyde dehydrogenase; NF-κB, nuclear factor kappa light chain enhancer of activated B cells; PG, prostaglandin; PGE2, prostaglandin E2; STAT, signal transducers and activators of transcription; TGF- β, transforming growth factor β; TNF, tumor necrosis factor; T-reg, T-regulatory cells.