Cleavage of the diketonitrile derivative of the herbicide isoxaflutole by extracellular fungal oxidases

Isoxaflutole is a herbicide activated in soils and plants to its diketonitr ile derivative, the active herbicide principle. The diketonitrile derivativ e undergoes cleavage to the inactive benzoic acid analogue. In this paper, it is established that an oxidative mechanism implicating two successive re actions in the presence of dimethyldioxirane can chemically initiate the cl eavage of the diketonitrile. It is also shown that two white rot strains, P hanerochaete chrysosporium and Trametes versicolor, are able to convert the diketonitrile to the acid when cultured in liquid media. This main metabol ite amounts to 24.6 and 15.1% of initial herbicide content after 12-15 days of culture. Another polar metabolite represents <3.7% of the parent compou nd amount during the same period. Oxidative enzymes produced by the fungi s how a time course similar to that of diketonitrile degradation. Purified la ccase (EC 1.10.3.2), in the presence of 2 mM 2,2'-azinobis(3-ethylbenzthiaz oline-6-sulfonic acid) acting as a redox mediator at pH 3 supports the reac tion with rates of 0.3-0.4 nmol h(-1) unit(-1).