TDO2 and Anxiety: A Heme flavoprotein cytosolic enzyme that catalyzes the first rate-limiting step of the kynurenic pathway- oxidative decarboxylation of L tryptophan with pyrrole ring cleavage, to produce NH3+CO2+ an unstable quinoxaline derivative that undergoes oxidative reduction with modification to resemble an HPL-like molecule. In a pseudomonas bacterium, the enzyme activity appeared to increase immediately after cessation of cell growth, responsible for anxiety-related behavior. TDO knockout mice had increased plasma levels of tryptophan, serotonin, and 5-HIAA in the midbrain and hippocampus