Endosulfan is a chlorinated cyclodiene pesticide used for over three decades in agriculture. Because it has been extensively applied directly to fields, it can be detected a considerable distance away from the original site of application. Contamination of drinking water and food, as well as detrimental effects to wildlife are important concerns (Siddique et al., 2003). The EPA allows endosulfan to be present in food no more than 0.1 to 2 ppm. i The FDA prohibits levels above 24 ppm in dry tea. High doses of endosulfan in humans primarily affect the central nervous system, resulting in respiratory depression, dry mouth, convulsions, and possibly death. Low doses have been shown to compromise the immune system, liver, kidneys and reproductive system in animals. This compound has been shown to be highly toxic with oral LD50 values ranging from 18 to 160 mg/kg in rats. For dermal exposure, LD50 values in rats ranged from 78 to 359 mg/kg (EXTOXNET).
Commercially available endosulfan exists as two diastereoisomers, alpha and beta, in a ratio of 7:3. The former is more volatile and thought to be more toxic (Siddique et al., 2003 and Lee et al., 2006). Thus, it is important to note that some microbial enzymes are specific to one isomer, or catalyze at different rates for each isomer (Kwon et al., 2005). For example, a Mycobacterium tuberculosis ESD enzyme degrades beta-endosulfan to the monoaldehyde and hydroxyether (depending on the reducing equivalent stoichiometry), but transforms alpha-endosulfan to the more toxic endosulfan sulfate (Sutherland et al., 2002a). However, oxidation of endosulfan or endosulfan sulfate by the monooxygenase encoded by ese in Arthrobacter sp. KW yields endosulfan monoalcohol (Weir et al., 2006). Both ese and esd proteins are part of the unique Two Component Flavin Dependent Monooxygenase Family, which require reduced flavin. They are conditionally expressed when no or very little sulfate or sulfite is available, and endosulfan is available to provide sulfur in these starved conditions (Sutherland et al., 2002b). The references include more details about isomer specificity and rates.
Alternatively, hydrolysis of endosulfan in some bacteria (Pseudomonas aeruginosa, Burkholderia cepaeia) yields the less toxic metabolite endosulfan diol (Kumar et al., 2006). Endosulfan can spontanteously hydrolyze to the diol in alkaline conditions, so it is difficult to separate bacterial from abiotic hydrolysis (Sutherland et al., 2002a). The diol can be converted to endosulfan ether (Hussein et al., 2007) or endosulfan hydroxyether (Lee et al., 2003) and then endosulfan lactone (Awasthi et al., 2003). Hydrolysis of endosulfan lactone yields endosulfan hydroxycarboxylate (Walse et al., 2003). These various branches of endosulfan degradation all result in desulfurization while leaving the chlorines intact, exhibiting the recalcitrance to bioremediation found in many organohalogen aromatics.
The following is a text-format endosulfan 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 (10k) format.
Endosulfan Endosulfan Endosulfan
Stenotrophomonas maltophilia Psueodmonas aeruginosa Mycobacterium sp. ESD
Rhodococcus erythropolis Burkholderia cepacia | |
Arthrobacter sp. KW | | |
/ | | | |
/ | endosulfan | A | | endosulfan
/ | monooxygenase I | | | monooxygenase II
/ | | | |
/ | endosulfan | | |
/ v sulfate hydrolase v | |
/ Endosulfan sulfate ----------------------> Endosulfan diol / |
/ | / | / |
| | / | / |
| | / | / |
endosulfan | | endosulfan endosulfan diol / | endosulfan diol / |
sulfate | | sulfate hydrolyase / | dehydrogenase / v
monooxygenase | | monooxygenase (cyclizing) / | / Endosulfan
| | / | / monoaldehyde
| v v | /
| Endosulfan hemisulfate Endosulfan | /
\ | ether | /
\ | \ | +----------+
\ | endosulfan \ | / endosulfan
\ | hemisulfate \ | / monooxygenase II
\ | sulfatase endosulfan \ | /
\ | ether \ | /
\ | monooxygenase \ | /
v v \ | /
Endosulfan monoalcohol | | |
v v v
Endosulfan
hydroxyether
|
|
| endosulfan
| hydroxyether
| dehydrogenase
|
v
Endosulfan
lactone
|
|
| endosulfan
| lactone
| lactonase
|
v
Endosulfan
hydroxycarboxylate
Page Author(s): Valerie Klema and Edward LaBelle
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