TDO2 and COVID-19: Tryptophan was diminished in mild and critical COVID-19 patients compared to uninfected controls, suggesting a disease-associated activation of tryptophan-consuming indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) that produce the kynurenic acid precursor kynurenine17.