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The hemoprotein indoleamine 2,3-dioxygenase (IDO) is the first and rate-limiting enzyme in mammalian tryptophan metabolism. It has received considerable attention in recent years, particularly due to its role in the pathogenesis of many diseases. Here, we report attempts to improve soluble expression and purification of hexahistidyl-tagged recombinant human IDO from Escherichia coli (EC538, pREP4, and pQE9-IDO). Significant formation of inclusion bodies was noted at the growth temperature of 37°C, with reduced formation at 30°C. The addition of the natural biosynthetic precursor of protoporphrin IX, δ-aminolevulinic acid (ALA), coupled with optimisation of IPTG induction levels during expression at 30°C and purification by nickel–agarose and size exclusion chromatography, resulted in protein with 1mol of heme/mol of protein and a specific activity of 160μmol of kynurenine/h/mg of protein (both identical to native human IDO). The protein was homogeneous in terms of electrophoretic analysis. Yields of soluble protein (3–5mg/L of bacterial culture) and heme content are greater than previously reported. [Copyright &y& Elsevier]