Dechlorination, enzymatic

Reductive reactions typically occur in anaerobic environments where there is an abundant supply of electron donors. Electron donors are typically of microbial origin, eg, porphyrins or cysteine, which sometimes leads to confusion regarding the nature, ie, chemical vs enzymatic, of the reductive reaction. By definition, all reductive reactions which are not enzymatically catalyzed are chemical. The most significant chemical reductive reaction is reductive dechlorination. 

The reductive dechlorination of chlorinated aromatics is more compHcated in that the initial dechlorination of more highly chlorinated compounds may be either chemical or enzymatic, eg, PGP, whereas the dechlorination of less chlorinated compounds or dechlorinated products is typically enzymatic. For example, the first dechlorination of 2,4-dichlorophenol (ortho position) can occur either chemically or enzymatically the second dechlorination (para position) is enzymatic (eq. 10). 

Van Dyke RA, Wineman CG. 1971. Enzymatic dechlorination. Dechlorination of chloroethanes and propanes in vitro. Biochem Pharmacol 20 463-470. 

The kinetics of the piperidino-defluorination reaction of l-fluoro-2,4-dinitrobenzene have been studied in non-aqueous reverse micelles consisting of ethylene glycol-AOT-/ -heptane or DMF-AOT-n-heptane. The reaction, which is not base catalysed, is accelerated when DMF, a non-hydrogen bond donor solvent, is used in the micelle core.22 Catalysis by human glutathione 5-transferase M la-la of the reaction of glutathione with 1-chloro- and l-fluoro-2,4-dinitrobenzenes has been investigated. Much stronger enzymatic catalysis was observed in the case of the dechlorination reaction than for the defluorination and a transition-state model was proposed 23... 

A glassy C electrode, which was immobilized with tyrosinase, was used for amperometric detection of phenol in a polyimide chip. Phenol was enzymatically converted to catechol, which was then oxidized to quinone during detection. Chlorophenol can also be detected, but this is achieved after a dechlorination step (to phenol) using a Mg/Pd metal catalyst [229]. 

DDT can be reductively dechlorinated to TDE (DDD) by anaerobic liver plus NADPH, dead tissues, and some microorganisms (Figure 8.17). It is not known whether the reaction proceeds enzymatically or nonenzymatically. 

Reductive Reactions. A number of pesticides are susceptible to reductive reactions under anaerobic conditions, depending on the substituents present on the molecule. Reductive reactions can be either chemically or enzymatically mediated. Because biologically generated reductants, eg, cysteine and porphyrins, are frequendy the electron donors for both chemical and enzymatic reactions, results from sterile controls are not necessarily conclusive in distinguishing between the two mechanisms. The only definitive means of distinguishing between chemical vs biological (enzymatic) reactions is to determine whether the reaction rate is consistent with enzyme kinetics. The most common reductive reactions are the reduction of nitro substituents and reductive dechlorination. 

Adenosine deaminase can be used for the enzymatic deamination and dechlorination of C6 substituted purine nucleosides -(see Houben-Weyl, Vol. 4/1 a, p1133), 9-[(2-hydroxy-ethoxy)methyl]adenineas well as 9-[(2-hydroxyethoxy)methyl]purine-2,6-diamine, the latter giving 9 in 87% yield. 

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