Hydrolytic dehalogenation of 4-chlorobenzoic acid by an Acinetobacter sp.

Acinetobacter sp. strain ST-1, isolated from garden soil, can mineralize 4-chlorobenzoic acid (4-CBA). The bacterium degrades 4-CBA, starting with dehalogenation to yield 4-hydroxybenzoic acid (4-HBA) under both aerobic and anaerobic conditions, suggesting that the dehalogenating enzyme in the strain is not an oxygenase; the enzyme may catalyze halide hydrolysis. To identify the oxygen source of the C(4)-hydroxy groups in the dehalogenation step, we used H(2)(18)O as the solvent under anaerobic conditions. When resting cells were incubated in the presence of 4-CBA and H(2)(18)O under a nitrogen gas stream, the hydroxy group on the aromatic nucleus of the 4-HBA produced was derived from water, not from molecular oxygen. This dehalogenation was hydrolytic, because analysis of the mass spectrum of the trimethylsilyl derivative of one of the metabolites, (18)O-labeled 4-HBA, showed that 80% of the C4-hydroxy groups were labeled with (18)O. Hydrolytic dehalogenation of 4-CBA in intact cells has not been reported earlier. To identify substrate specificity, we next examined the ability of the strain to dehalogenate 4-CBA analogues and dichlorobenzoic acids. The results of metabolite analysis by high-pressure liquid chromatography showed that the strain dehalogenated 4-bromobenzoic acid and 4-iodobenzoic acid, yielding 4-HBA, suggesting that these compounds could be further degraded and mineralized by the strain via the beta-ketoadipate pathway, as occurs with 4-CBA. This strain, however, did not dehalogenate 4-fluorobenzoic acid, 2- and 3-chlorobenzoic acids, or 2,4-, 3,4-, and 3,5-dichlorobenzoic acids during 4 days of incubation, implying that the dehalogenating enzyme of the strain has high substrate specificity.