Enantioselective Uptake and Degradation of the Chiral Herbicide Dichlorprop [( RS )-2-(2,4-Dichlorophenoxy)propanoic acid] by Sphingomonas herbicidovorans MH

Abstract

ABSTRACT Sphingomonas herbicidovorans MH was able to completely degrade both enantiomers of the chiral herbicide dichlorprop [( RS )-2-(2,4-dichlorophenoxy)propanoic acid], with preferential degradation of the ( S ) enantiomer over the ( R ) enantiomer. These results are in agreement with the recently reported enantioselective degradation of mecoprop [( RS )-2-(4-chloro-2-methylphenoxy)propanoic acid] by this bacterium (C. Zipper, K. Nickel, W. Angst, and H.-P. E. Kohler, Appl. Environ. Microbiol. 62:4318–4322, 1996). Uptake of ( R )-dichlorprop, ( S )-dichlorprop, and 2,4-D (2,4-dichlorophenoxyacetic acid) was inducible. Initial uptake rates of cells grown on the respective substrate showed substrate saturation kinetics with apparent affinity constants ( K t ) of 108, 93, and 117 μM and maximal velocities ( V max ) of 19, 10, and 21 nmol min −1 mg of protein −1 for ( R )-dichlorprop, ( S )-dichlorprop, and 2,4-D, respectively. Transport of ( R )-dichlorprop, ( S )-dichlorprop, and 2,4-D was completely inhibited by various uncouplers and by nigericin but was only marginally inhibited by valinomycin and by the ATPase inhibitor N , N ′-dicyclohexylcarbodiimine. Experiments on the substrate specificity of the putative transport systems revealed that ( R )-dichlorprop uptake was inhibited by ( R )-mecoprop but not by ( S )-mecoprop, ( S )-dichlorprop, or 2,4-D. On the other hand, the ( S )-dichlorprop transport was inhibited by ( S )-mecoprop but not by ( R )-mecoprop, ( R )-dichlorprop, or 2,4-D. These results provide evidence that the first step in the degradation of dichlorprop, mecoprop, and 2,4-D by S. herbicidovorans is active transport and that three inducible, proton gradient-driven uptake systems exist: one for ( R )-dichlorprop and ( R )-mecoprop, another for ( S )-dichlorprop and ( S )-mecoprop, and a third for 2,4-D.