Polyhalogenated compounds reductive metabolism

The refractory nature of some pollutants, notably, persistent polyhalogenated compounds, has raised problems of bioremediation of contaminated sites (e.g., sediments and dumping sites). There has been interest in the identification, or the production by genetic manipulation, of strains of microorganisms that can metabolically degrade recalcitrant molecules. For example, there are bacterial strains that can reductively dechlorinate PCBs under anaerobic conditions. 

Reductive biotransformations of several compounds such as polyhalogenated, keto, nitro and azo derivatives, are catalysed by a variety of enzymes which differ according to the substrates and the species. The liver cytochrome P-450-dependent drug metabolizing system is capable of reducing Af-oxide, nitro and azo bonds, whereas the cytosolic nitrobenzene reductase activity is mainly due to cytochrome P-450 reductase, which transforms nitrobenzene into its hydroxylamino derivative. NADPH cytochrome c reductase is also able to catalyse the reduction of nitro compounds. These metabolic conversions may also be brought about by gastrointestinal anaerobic bacteria. 

The mechanism of reductive dehalogenation of polyhalogenated compounds by microsomal cytochrome P450 has been studied in detail [22]. The main products of the in vitro metabolism of hexa- and pentachloroethane were tetra- and trichloroethene, respectively. In this case, the reductive dehalogenation probably proceeds by two sequential one electron reductions forming first a radical and then a carbanion. The carbanion may undergo... 

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