KYAT1 and depressive disorder: Moreover, previous studies confirmed that depression may be associated with the insufficient of tryptophan and serotonin, as well as increased activity of 2,3-dioxygenase tryptophan (TDO) and indoleamine 2,3-dioxygenase, but decreased activity of kynurenine aminotransferases I/II [4,8,9,11].