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Strain Tl is a facultative bacterium that is capable of anaerobic toluene degradation under denitrifying conditions. While 80% of the carbon from toluene is either oxidized to carbon dioxide or assimilated into cellular carbon, a significant portion of the remainder is transformed into two dead-end metabolites. These metabolites were produced simultaneous to the mineralization of toluene and were identified as benzylsuccinic acid and benzylfumaric acid. Identification was based on comparison of mass spectra of the methyl esters of the metabolites and authentic compounds that were chemically synthesized. Strain Tl is also capable of o-xylene transformation during growth on toluene. o-Xylene does not serve as a source of carbon and is not mineralized. Rather, it is transformed to analogous dead-end metabolites, (2-methylbenzyl)-succinic acid and (2-methylbenzyl)-fumaric acid. o-Xylene transformation also occurred during growth on succinic acid, which suggests that attack of the methyl group by succinyl-coenzyme A is a key reaction in this transformation. We reason that the main pathway for toluene oxidation to carbon dioxide involves a mechanism similar to that for the formation of the metabolites and involves an attack of the methyl group of toluene by acetyl-coenzyme A. Strain Ti is a denitrifying bacterium that has been shown to oxidize toluene under anaerobic conditions primarily to carbon dioxide and biomass (8). This organism is also capable of o-xylene transformation during growth on toluene; however, o-xylene cannot serve as a sole source of energy or cellular carbon. Metabolites were formed from both toluene and o-xylene during their transformation and were not further metabolized. These dead-end metabolites were determined not to be benzyl alcohol, benzaldehyde, benzoic acid, cresol, hydroxybenzyl alcohol, hydroxybenzaldehyde, hydroxybenzoic acid, methylbenzyl alcohol, tolualdehyde, or toluic acid (8). Since the metabolites were not further degraded, it was reasoned that they were not intermediates in the pathway of toluene oxidation to carbon dioxide. However, since a significant portion of toluene carbon was transformed to these metabolites, investigation of their formation was warranted. The toluene-dependent transformation of o-xylene is significant because o-xylene is relatively recalcitrant to biodegradation under both aerobic (2) and anaerobic (8) conditions. However, incomplete transformations have a potentially negative side since the metabolites formed may be more toxic than the original chemical that was the source of contamination. Therefore, identification of these metabolites can provide information on their toxicity, their formation, and how to initiate their mineralization.

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