An oxygen-insensitive intracellular enzyme that is responsible for the decolorization of azo dyes was purified from Escherichia coli CD-2. The molecular weight of the purified enzyme was estimated as 27,000 +/- 500 Da. Protein identification indicated that the enzyme had high sequence homology with E. coli K12 quinone reductase, and the enzyme was proved to have both azoreductase and quinone reductase activity. With methyl red as substrate, the optimal pH value and temperature were 6.5 and 37 degrees C, respectively. The enzyme was stable under different physiochemical conditions. The azoreductase activity was restrained by SDS and was almost completely inhibited by Co2+ and Hg2+. K-m and V-max values were 0.18 mM and 8.12 U mg(-1) of protein for NADH and 0.05 mM and 6.46 U mg(-1) of protein for methyl red, respectively. The purified enzyme could efficiently decolorize methyl red with both NADH and NADPH as electron donors. (C) 2011 Elsevier Ltd. All rights reserved.