2-Hydroxy-6-oxo-6-(2'-aminophenyl)-hexa-2,4dienoic acid [6-(2'-aminophenyl)-HODA] hydrolase, involved in carbazole degradation by Pseudomonas resinovorans strain CA10, was purified to near homogeneity from an overexpressing Escherichia coli strain. The enzyme was dimeric, and its optimum pH was 7.0-7.5. Phylogenetic analysis showed the close relationship of this enzyme to other hydrolases involved in the degradation of monocyclic aromatic compounds, and this enzyme was specific for 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (6-phenyl-HODA), having little activity toward 2-hydroxy-6-oxohepta-2,4-dienoic acid and 2-hydroxymuconic semialdehyde. The enzyme had a Km of 2.51 microM and k(cat) of 2.14 (s(-1)) for 6-phenyl-HODA (50 mM sodium phosphate, pH 7.5, 25 degrees C). The effect of the presence of an amino group or hydroxyl group at the 2'-position of phenyl moiety of 6-phenyl-HODA on the enzyme activity was found to be small; the activity decreased only in the order of 6-(2'-aminophenyl)-HODA (2.44 U/mg) > 6-phenyl-HODA (1.99 U / mg) > 2-hydroxy-6-oxo-6-(2'-hydroxyphenyl)-hexa-2,4-dienoic acid (1.05 U/mg). The effects of 2'-substitution on the activity were in accordance with the predicted reactivity based on the calculated lowest unoccupied molecular orbital energy for these substrates.