Reductive dechlorination of DDT

Under anaerobic conditions, p,p -DDT is converted to p,p -DDD by reductive dechlorination, a biotransfonnation that occurs postmortem in vertebrate tissues such as liver and muscle and in certain anaerobic microorganisms (Walker and Jefferies 1978). Reductive dechlorination is carried out by reduced iron porphyrins. It is carried out by cytochrome P450 of vertebrate liver microsomes when supplied with NADPH in the absence of oxygen (Walker 1969 Walker and Jefferies 1978). Reductive dechlorination by hepatic microsomal cytochrome P450 can account for the relatively rapid conversion of p,p -DDT to p,p -DDD in avian liver immediately after death, and mirrors the reductive dechlorination of other organochlorine substrates (e.g., CCI4 and halothane) under anaerobic conditions. It is uncertain to what extent, if at all, the reductive dechlorination of DDT occurs in vivo in vertebrates (Walker 1974). 

The nature of the radioactivity in the water, soil and fish from the carbon-14 DDT experiment was examined by thin-layer chromatography as shown in Figure 5. The radioactivity in the water was very polar in nature and did not migrate appreciably from the origin. About 78% of the radioactivity in the soil was extracted with methanol. The major metabolite in the extractable fraction was DDD which represented 33% of the total radioactivity. The reductive dechlorination of DDT to DDD is a known pathway under anaerobic conditions and has been shown to be due to microbial metabolism (5). Since carbon-14 DDT was incor-... 

Stromberg, J.R., Wnuk, J.D., Pinlac, R.A.F. and Meyer, G.J. (2006) Multielectron transfer at heme-functionalized nanocrystalline TiCU Reductive dechlorination of DDT and CCI4 forms stable carbene compounds. Nanoletters 6, 1284-1286. 

Figure 7. A possible mechanism for the reductive dechlorination of DDT to DDDy adapted from a proposal by Glass (38). The p-chloro-phenyl groups are represented by R.

The iron-mediated reductive dechlorination of DDT has been observed to occur by both hydrogenolysis and dehydrodehalogenation to give DDD and DDE, respectively (Glass, 1972 Zoro et al., 1974). 

Reductive dechlorination in anaerobic sediments also caused the conversion of pp DDT to p,p -DDD. Sediment half-lives of pp DDT were estimated to be between 14 and 21 years in Lake Ontario sediments with D order kinetics (Oliver et al., 1989) thus rates of degradation in sediments could be considered to interpret historical deposition. For example, the rate of reductive dechlorination of DDT in sediment cores was calculated by plotting In (DDT/ DDT + DDD) vs. sediment age as inferred by Pb (Oliver et al., 1989). Muir et al. (1995) found that the proportion of DDD/DDD + DDT increased downcore in many lakes along a mid-continental transect from Northwestern Ontario to Ellesmere Island (Canadian High Arctic). The half-life for anaerobic conversion of DDT to DDD was <20 years in most lakes (assuming a constant ratio of DDD/DDT input to sediments). 

Walker, C.H. (1969). Reductive dechlorination of p,p -DDT by pigeon liver microsomes. Life Science , 111-115. 

Walker, C.H. and Jefferies, D.J. (1978). The post mortem reductive dechlorination of p,p -DDT in avian tissues. Pesticide Biochemistry and Physiology 9, 203-210. 

The degradation of DDT by organisms designated Aerobacter aerogenes (possibly Klebsiella aerogenes) (Wedemeyer 1967) (Figure 2.6), and the partial reductive dechlorination of methoxychlor by K. pneumoniae (Baarschers et al. 1982). 

Assaf-Anid, N., Hayes, K. F. Vogel, T. M. (1994). Reductive dechlorination of carbon tetrachloride by cobalamin(II) in the presence of dithiothreitol mechanistic study, effect of redox potential and pH. Environmental Science Technology, 28, 246-52. Ballard, T. M. (1971). Role of humic carrier substance in DDT movement through forest soil. Soil Science Society of America Proceedings, 35, 145-7. 

The reductive dechlorination of alachlor, metolachlor, and DDT, three important pesticides, was observed to occur by hydrogenolysis [21, 45]. First-order degradation rate constants were reported to be 0.12 and 0.10 h 1 for 10 mg/1 solutions of alachlor and metochlor, respectively. 

Rusling and coworkers have carried out extensive studies of the use of electrogenerated cobalt(I) complexes (including cobalt(I) salen, vitamin Bi2s, and cobalt(I) phthalo-cyanine) as catalysts both in homogeneous phase and in bicontinuous microemulsions [384] for the reductions of 1,2-dibromoethane and 1,2-dibromobutane [385], the debromi-nation of alkyl vicinal dibromides [386], the dechlorination of DDT [387], the reductions of 1-bromobutane, 1-bromododecane, and ran5-l,2-dibromocyclohexane [388,389], and the reduction of benzyl bromide [390]. 

The groups on the phenyl rings appear to have some effect on the reductive dechlorination of the trichloroethane. Thus, o,p -DDT is de-chlorinated reductively to o,p -DDD by mechanisms and rates similar to the reductive dechlorination of p,p -DDT 17, 25). In contrast, Mendel et al. 17) were unable to obtain reductive dechlorination when the phenyl chlorine atoms of p,p -DDT were replaced by ethyl or methoxy groups. It is interesting, however, that they were unable to recover completely the methoxychlor. This indicates anaerobic degradation of this compound by some other route. 

DDT is slowly converted in vivo by reductive dechlorination to DDD and by further dechlorinations to 4,4 -dichlorodiphenylacetic acid [83-05-6] (DDA), the predominant excretory metaboUte. Anaerobically, it may form 4,4 -dichlorodiphenyiacetonitrile [20968-04-1] (DDCN). However, most DDT that enters the environment is sequestered as DDE, which is ubiquitously present in the body Hpids of invertebrate and vertebrate animals. In humans. 

Transformation of DDT to DDD by reductive dechlorination has been demonstrated in a number of aquatic plants, although the reaction appears to be abiotic mediated by some component of the plants (Garrison et al. 2000). 

Soil. p,//-DDD and p./Z-DDE are the major metabolites of /5,//-DDT in the environment (Metcalf 1973). In soils under anaerobic conditions, /5,//-DDT is rapidly converted to p,//-DDD via reductive dechlorination (Johnsen, 1976) and very slowly to p,//-DDE under aerobic conditions (Guenzi and Beard, 1967 Kearney and Kaufman, 1976). The aerobic degradation of p,pD yY under flooded conditions is very slow with p,//-DDE forming as the major metabolite. Dicofol was also detected in minor amounts (Lichtenstein et al., 1971). In addition to p./Z-DDD and /5,//-DDE, 2,2-bis(/5-chlorophenyl)acetic acid (DDA), bis(jo-chlorophenyl)methane (DDM), /5,/y dichlorobenzhydrol (DBH), DBF, and p-chlorophenylacetic acid (PCPA) were also reported as metabolites of /5,//-DDT in soil under aerobic conditions (Subba-Rao and Alexander, 1980). 

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