1,1,1-Trichloro-2,2-bis-(4'-chlorophenyl)ethane (DDT), the first of the chlorinated organic insecticides, is a widely distributed and persistent xenobiotic contaminant in the environment. DDT is not metabolized very rapidly by animals; instead, it is deposited and stored in the fatty tissues. The biological half-life of DDT is about eight years; that is, it takes about eight years for an animal to metabolize half of the amount it assimilates. If ingestion continues at a steady rate, DDT builds up within the animal over time. Despite being banned in the United States, DDT continues to be widely used in many developing nations. This page shows only its aerobic pathway. The anaerobic degradation of DDT is documented elsewhere in the EAWAG-BBD.
The chemical structure of DDT, including aromatic and alicyclic moieties, offers a variety of possibilities for biochemical attack. Several aerobic pathways for DDT metabolism in different organisms were suggested (Quensen et al., 1998, Hay et al., 1998, Nadeau et al., 1994). Compounds in brackets were not identified but postulated based on other studies, such as those of the Biphenyl Pathway.
The steps labeled A, B, C, D, E are mutiple steps whose intermediates are not identified yet. All descriptions as follows were proposed based on other similar experiments. In A, DDE is attacked by a dioxygenase at the ortho and meta positions. Such an attack would give rise to a 2,3-dihydrodiol-DDE intermediate. In B and D, 2-(4'-Chlorophenyl)-3,3-dichloropropenoate may proceed via decarboxylation to yield 1,1-dichloro-(4'-chlorophenyl)ethane. The latter will undergo oxidation of the aliphatic side chain to yield 1,1-dichloro-(4'-chlorophenyl)ethanol which is further oxidized to yield 4-Chloroacetophenone. The terminal methyl group of 1,1-dichloro-(4'-chlorophenyl)ethane may also undergo oxidation to yield phenylacetic acid. In C, the transformation of 4-Chloroacetophenone to 4-Chlorobenzaldehyde may be via complete oxidation and subsequent decarboxylation of the terminal methyl group. In E, the ring-cleavage product would be further degraded to either a C-6- or C-5-chlorinated acid, depending on where the hydrolytic cleavage takes place.
The following is a text-format DDT pathway map. Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions. This map is also available in graphic (21K) format.
1,1,1-Trichloro-2,2- 1,1,1-Trichloro-2,2- bis(4'-chlorophenyl)ethane bis(4'-chlorophenyl)ethane (DDT) (DDT) Microbial Consortium Ralstonia eutropha A5 | | | from the | DDT dehydrochlorinase | +--- Anaerobic Pathway | DDT 2,3-dioxygenase | | | V V | 1,1-Dichloro-2,2- V bis(4'-chlorophenyl)- [cis-2,3-Dihydrodiol DDT] ethylene (DDE) | | | Microbial Consortium | Pseudomonas acidovorans M3GY | +------------+------------+ | cis-2,3-dihydrodiol DDE de- | V A | DDT dehydrogenase halogenase | | | | | | V V | 1-Chloro-2,2-bis- 1,1-Dichloro-2- V (4'-chlorophenyl)- (dihydroxy-4'-chlorophenyl)- [2,3-Dihydroxy DDT] ethylene (DDMU) 2-(4-chlorophenyl)ethylene | | | | | | 1,1-dichloro-2-(dihy- | | | droxy-4'-chlorophenyl)- | 2,3-dihydroxy DDT | | 2-(4'-chlorophenyl)- | 1,2-dioxygenase V | ethylene 1,2-dioxygenase | to the | | Anaerobic Pathway V V 6-Oxo-2-hydroxy- 6-Oxo-2-hydroxy- 7-(4'-chlorophenyl)- 7-(4'-chlorophenyl)- 3,8,8-trichloroocta- 3,8,8,8-tetrachloroocta- 2Z,4Z,7-trienoate 2Z,4Z-dienoate | | | 6-oxo-2-hydroxy-7-(4'- | | chlorophenyl)-3,8,8- | | trichloroocta-2Z,4Z,7- | | trienoate hydrolase | | | V V E 2-(4'-Chlorophenyl)-3,3- | +-------------- dichloropropenoate | | | | | | | V D V B | | | V | | +----> 4-Chlorobenzoate V V | | 4-Chlorophenyl- 4-Chloroacetophenone | | acetate | | 4-chloro- | | | | benzaldehyde | | V C | dehydrogenase | | | | | | | | V V V | to the to the 4-Chlorobenzaldehyde --+ 2,4-Dichlorobenzoate L-Tyrosine Pathway Pathway
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