Degradation of chlorophenoxyacetic acids by the lignin-degrading fungus Dichomitus squalens

Abstract

We have examined the degradation of 14C ring-and side-chain-labelled 2,4,5-trichlorophenoxyacetic acid by Dichomitus squalens and Phanerochaete chrysosporium. The effects of Mn2+ on the degradation of these radiolabelled substrates by D. squalens and the effect of nitrogen limitation on their degradation by P. chrysosporium suggested that in both fungi, side-chain cleavage was catalysed by a mechanism independent of the lignin degradation system, whereas the degradation of the aromatic ring was dependent on the lignin degradative system. Using unlabelled substrates, a pathway for the degradation of chlorophenoxyacetic acids was elucidated in D. squalens. Time courses for the degradation of unlabelled chlorophenoxyacetic acids by D. squalens demonstrated that the corresponding chlorophenol was the initial product formed. The chlorophenol intermediate was xylosylated to form the chlorophenolxyloside. In turn, the chlorophenolxyloside could be hydrolysed by an intracellular p-xylosidase to regenerate the chlorophenol. The chlorophenol product of the xylosidase reaction was oxidatively dechlorinated to form 2-chloro-p-benzoquinone which could undergo subsequent further dechlorination and ring-opening reactions, as has been reported previously for P. chrysosporium.