DETOXIFICATION OF 2,4-DICHLOROPHENOL BY A LACCASE IMMOBILIZED ON SOIL OR CLAY

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Abstract
We investigated the ability of an immobilized fungal laccase from Trametes versicolor to transform 2,4-dichlorophenol (2,4-DCP), an intermediate of pesticide degradation. Immobilization supports were a silt loam soil and several clays, kaolinite, and two types of smectite, referred to as montmorillonite 1 and montmorillonite 2. The majority of the added laccase activity was immobilized to these supports, using 3-aminopropyltriethoxysilane and glutaraldehyde. The catalytic properties of these laccase-clay and laccase-soil complexes toward the substrate, 2,4-DCP, were assessed using radiolabeled tracer and chromatographic techniques. Incubation of 2,4-DCP with laccase caused the formation of oligomers, which are insoluble in aqueous solutions and can, therefore, be removed. The efficiency of the laccase immobilized on kaolinite and soil to remove 2,4-DCP from solution was similar to that of the free enzyme (approximately 95% of the added 14C-2,4-DCP was removed) and greater than that of the enzyme immobilized on montmorillonites 1 and 2 (69 and 42%, respectively). For these last two supports, however, enzymatic removal may not have been responsible for the entire loss of 2,4-DCP, as adsorption on montmorillonites 1 and 2 also contributed to the observed decreases in 2,4-DCP. A noteworthy advantage of the immobilized laccase over the free enzyme was that it could be separated from the reaction mixture and used repeatedly. After repeated 2-h incubation cycles of the immobilized enzyme with 14C-2,4-DCP, the laccase-kaolinite, laccase-montmorillonite 1, and laccase-soil complexes exhibited only a slight decrease in the ability to remove 2,4-DCP. The possibility of nonenzymatic removal of 2,4-DCP was excluded in experiments performed under anaerobic conditions, in which laccase activity was completely inhibited. The effect of a proteolytic enzyme on the free and immobilized laccase was also tested, and, as a result of their resistance to proteolysis, the immobilized enzyme catalyzed the removal of greater amounts of 2,4-DCP than the free enzyme. This stability and certain other characteristics, such as their reusability and their ability to transform xenobiotics as efficiently as the soluble enzyme, should make these laccase complexes useful catalysts of detoxification reactions in soil.

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