Phorate is a powerful pesticide effective against insects, mites, and nematodes. It is a systemic insecticide that acts by inhibiting cholinesterases, enzymes involved in transmitting nerve impulses. It is not recommended for use by the homeowners because it is extremely toxic and persistent. Because of its persistence in soil it is most useful in controlling soil-dwelling nematodes (microscopic roundworms that suck plant roots and transmit disease to them).
Chemically, it is an organophosphate, O,O-diethyl-S-(ethylthio)methylphosphorodithioate. Like all organophosphates, it is related to the nerve gases and is among the most toxic of all pesticides to vertebrates, including humans (Rani et al., 2009). It is among the most widely used pesticides even though it is highly toxic. According to the EXTOXNET Phorate Pesticide Information Profile, single-time exposure at high doses can interfere with cholinesterase activity leading to numbness, headache, dizziness, nausea, a difficulty breathing, and slow heartbeat and if the dose is too high, potentially death. Long-term, low-dose exposure primarily targets the nervous system, with little evidence supporting any teratogenic, carcinogenic, or mutagenic effects.
Degradation of phorate occurs in two branches. The left branch is a two-step oxidation of the ethylthio sulfur (Laveglia and Dahm, 1977). In the right branch, Ralstonia eutropha strain AAJ1 degrades a high percentage of phorate to phosphate and sulfate anions, with the less toxic diethyl dithiophosphate as one intermediate (Rani et al., 2009). The reaction intermediates diethylthiophosphate, ethylthiophosphate and coproduct ethanol are postulated, though diethylthiophospate is observed in parathion degradation. The initial reaction in the right branch, resulting in the formation of diethyl dithiophosphate, is not well characterised.
The following is a text-format phorate pathway map. An organism which can initiate the pathway is 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 (14k) format.
Phorate Phorate
Ralstonia eutropha sp. Ralstonia eutropha sp.
AAJ1 AAJ1
| |
| phorate | A
| monooxygenase |
| |
v v
Phorate sulfoxide Diethyl dithiophosphate
| |
| phorate sulfoxide | diethyl dithiophosphate
| monooxygenase | oxidoreductase
| |
v v
Phorate sulfone +------- [Diethylthiophosphate] <------ from the Parathion Pathway
| |
| diethyl- |
| thiophosphate | diethylthiophosphate
| oxidoreductase | phosphodiesterase
v |
+-----> Diethylphosphate +-----------------------------+
| | |
| | |
from the v |
Parathion Pathway Ethylthiophosphate |
| |
| ethylthiophosphate |
| oxidoreductase |
| |
v |
[Ethylphosphate] |
| |
| ethylphosphate |
| phosphomonoesterase |
| v
+-----------------------> [Ethanol]
| |
| |
v v
Phosphate Intermediary
Metabolism
(KEGG)
Page Author(s): Denise Hauge, Sujana Mittapalli
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