Chapter 11
Transformation of Nitroaromatic Pesticides and Related Xenobiotics by Microorganisms and Plants 1
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Robert M. Zablotowicz, Robert E. Hoagland, Hung Lee , T. Alber , Jack T. Trevors , J. Christopher Hall , and Martin A. Locke 2
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1Southern Weed Science Research Unit, Agricultural Research Service, U.S. Department of Agriculture, Stoneville, MS 38776 Department of Environmental Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada Fachhochule Weinstephan, Fachbereich Biotechnolgie, Friesing, Germany 2
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Nitroaromatic compounds have a range of uses including applications as pesticides and explosives. Certain nitroaromatics are common contaminants in soil and water due to their recalcitrance and wide use. Bacteria, fungi, and plants possess a wide array of enzymatic processes involved in transformation of these compounds. Bacterial oxidative mechanisms include: flavin monooxygenase mediated-nitrite release from p-nitrophenol, 2,4dinitrophenol, dinitrocresol, and dioxygenase-mediated nitrite release from 2,6-dinitrophenol and nitrotoluene. Aromatic nitroreductases are ubiquitous among bacteria, certain fungi and plants. Oxidative pathways facilitate extensive degradation of nitroaromatics, however enzymes in these pathways typically have a more specific substrate range than nitroreductases. Elimination of reduced nitro groups by partial nitroreductive processes aids in subsequent metabolism of these molecules. Reductive catabolic pathways for 2,4-dinitrophenol and picric acid involve initial hydrogenation reactions. Specific plant transformations include: glutathione S-transferase-mediated nitrite release, nitroreductase, and ferredoxin-NADP oxidoreductase-mediated nitrite liberation. The biochemistry, physiology, and ecology of nitroaromatic metabolism are reviewed, emphasizing systems for xenobiotic detoxification.
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© 2001 American Chemical Society In Pesticide Biotransformation in Plants and Microorganisms; Hall, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
195 Nitroaromatic compounds are used in the synthesis of pesticides, dyes, explosives, pharmaceuticals and are of concern as environmental pollutants. Primary nitroaromatic pesticides include: dinitroaniline herbicides, e.g., pendimethalin [JV-(1ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] and trifluralin [2,6-dinitro-A/ ^Vdipropyl-4-(trifluoromethylbenzenamine)]; dinitrophenol herbicides, e.g., dinoseb (46-dinitro-0-sec-butylphenol) and 4,6-dinitrocresol (DNOC); nitrodiphenylether herbicides, e.g., acifluorfen, {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}; the fungicide, pentachloronitrobenzene (PCNB); and the insecticide, parathion (0,0-diethy1-0-4-nitrophenol-phosphorothioate). In microorganisms, enzymes from diverse pathways metabolize nitroaromatic compounds, depending on the chemical structure and the microbial species (/). Likewise, numerous pathways for metabolism of nitroaromatic compounds have been described in plants. We will present some of our research on oxidative and reductive metabolism of nitroaromatic compounds by bacteria, and provide an overview on other aspects of plant and microbial transformation of nitroaromatic pesticides and related xenobiotics.
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Microbial Oxidative Pathways of Nitroaromatic Metabolism Oxidative biotransformations of numerous nitroaromatic xenobiotics have been described in diverse genera of aerobic bacteria, e.g., Arthrobacter, Bacillus, Comamonas, Flavobacterium, Moraxella, Nocardia, Pseudomonas, and Sphingomonas (Table I). With many compounds, especially more polar nitrobenzoic acids (2) and nitrophenols (3), the nitro group is enzymaticaly removed by a monooxygenase or dioxygenase, prior to ring cleavage. However, with some substrates, the nitro-group is not released until after ring cleavage, as in nitrobenzene metabolism by Pseudomonas putida (4). Certain bacteria, i.e., Nocardia sp. strain T W 2 can degrade a nitrophenols via several pathways (5). When T W 2 was grown (induced) on phenol, m-nitrophenol (MNP), or p-cresol, the intermediate of p~ nitrophenol (PNP) metabolism was 4-nitrocatechol. However, when T W 2 was grown on PNP, hydroquinone was the intermediate. Although numerous oxidative transformations have been implicated in the literature, the enzymes responsible for these processes have been only rarely isolated and characterized. For example, Gunderson and Jensen (6) isolated an Arthrobacter simplex strain that metabolizes D N O C , with 3-methyl-5-nitrocatechol and 2,3,5-trihydroxytoluene as intermediates, following sequential nitrite liberation.
Monooxygenases Oxidative removal of nitro groups from
