TDO2, also known as TDO, is a ferrous heme enzyme that catalyzes the first and rate-limiting step in the kynurenine pathway, the major pathway of tryptophan metabolism. It incorporates oxygen into the indole moiety of tryptophan. It has a broad specificity towards tryptamine and derivatives including D- and L-tryptophan, 5-hydroxytryptophan and serotonin. Recombinant human TDO2 protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.