IDO2 and major depressive disorder: When the organism is in a state of inflammation or stress, the activity of the important rate-limiting enzyme in the Kyn metabolism pathway, indoleamine 2, 3-dioxygenase (IDO) increases, which promotes the metabolism of Trp to Kyn, resulting in insufficient content of Trp, a precursor substance for the synthesis of 5-HT in the brain, affecting the production of 5-HT and thereby exacerbating depression (Zhang et al., 2020).