BIODEGRADATION OF NITROAROMATIC COMPOUNDS

Nitroaromatic compounds are released into the biosphere almost exclusi vely from anthropogenic sources. Some compounds are produced by incomp lete combustion of fossil fuels; others are used as synthetic intermed iates, dyes, pesticides, and explosives. Recent research revealed a nu mber of microbial systems capable of transforming or biodegrading nitr oaromatic compounds. Anaerobic bacteria can reduce the nitro group via nitroso and hydroxylamino intermediates to the corresponding amines. Isolates of Desulfovibrio spp. can use nitroaromatic compounds as thei r source of nitrogen. They can also reduce 2,4,6-trinitrotoluene to 2, 4,6-triaminotoluene. Several strains of Clostridium can catalyze a sim ilar reduction and also seem to be able to degrade the molecule to sma ll aliphatic acids. Anaerobic systems have been demonstrated to destro y munitions and pesticides in soil. Fungi can extensively degrade or m ineralize a variety of nitroaromatic compounds. For example, Phaneroch aete chrysosporium mineralizes 2,4-dinitrotoluene and 2,4,6-trinitroto luene and shows promise as the basis for bioremediation strategies. Th e anaerobic bacteria and the fungi mentioned above mostly transform ni troaromatic compounds via fortuitous reactions. In contrast, a number of nitroaromatic compounds can settle as growth substrates for aerobic bacteria. Removal or productive metabolism of nitro groups can be acc omplished by four different strategies. (a) Some bacteria can reduce t he aromatic ring of dinitro and trinitro compounds by the addition of a hydride ion to form a hydride-Meisenheimer complex, which subsequent ly rearomatizes with the elimination of nitrite. (b) Monooxygenase enz ymes can add a single oxygen atom and eliminate the nitro group from n itrophenols. (c) Dioxygenase enzymes can insert two hydroxyl groups in to the aromatic ring and precipitate the spontaneous elimination of th e nitro group from a variety of nitroaromatic compounds. (d) Reduction of the nitro group to the corresponding hydroxylamine is the initial reaction in the productive metabolism of nitrobenzene, 4-nitrotoluene, and 4-nitrobenzoate. The hydroxylamines undergo enzyme-catalyzed rear rangements to hydroxylated compounds that are substrates for ring-fiss ion reactions. Potential applications of the above reactions include n ot only the biodegradation of environmental contaminants, but also bio catalysis and synthesis of valuable organic molecules.