PROPAPHOS

The Uses of PROPAPHOS

Propaphos is used to control rice hoppers and stem borers in rice.

The Uses of PROPAPHOS

Propaphos acts as a pesticide

Definition

ChEBI: Propaphos is an organophosphate insecticide and a dialkyl aryl phosphate. It has a role as an EC 3.1.1.7 (acetylcholinesterase) inhibitor and an agrochemical. It is functionally related to a 4-(methylsulfanyl)phenol.

Metabolic pathway

As with other organophosphorus insecticides containing a methylthiosubstituted phenyl group (fenamiphos, fenthion and sulprofos), the principal route of metabolism of is via thiooxidation to the sulfoxide and sulfone. The two pathways for the metabolism of propaphos are degradation by hydrolysis followed by oxidation to give the thiooxidation products of 4-thiomethyphenol and activation yielding propaphos sulfoxide and sulfone. Similar pathways exist in both animals and plants. Phase II metabolism involves the conjugation of the oxidised 4thiomethyl phenols.

Degradation

Propaphos is stable in neutral and acidic solutions but it is slowly hydrolysed under alkaline conditions (PM). The photolysis of propaphos has been studied under UV and sunlight irradiation in aqueous solution (Fujii et al., 1979) and by xenon lamp irradiation in methanol and aqueous solution (Koshioka et al., 1986).
Under UV irradiation at 254 or 365 nm propaphos was rapidly decomposed to CO₂. Under sunlight irradiation, the photolysis products were both oxidative and hydrolytic forming propaphos sulfoxide (2), propaphos sulfone (3), 4-(methy1thio)phenol (4), 4-(methylsulfinyl)- phenol (5) and 4-(methylsulfonyl)phenol (6) together with unidentified polar products (Fujii et al., 1979). Using xenon lamp irradiation of propaphos and its two thiooxidation products, Koshioka et al. (1986) showed that the photodecomposition of propaphos (1), propaphos sulfoxide (2) and sulfone (3) was rapid in the near UV (>280 nm) in aqueous solution, with the main photoproducts being due to oxidation and hydrolysis. Using GC-MS analysis the following eight metabolites were identified from the aqueous photolysis of propaphos: the thiooxidation products propaphos sulfoxide (2) and propaphos sulfone (3) and the hydrolytic products 4-(methylthio)phenol (4), 4(methylsulfinyl) phenol (5) and 4-(methylsulfonyl)phenol (6). Additionally, the methylthio group was lost, probably via the sulfone to give dipropyl phenyl phosphate (7). Two minor metabolites were identified as 1,4- benzenediol (8) and 4-hydroxythiophenol (9). Possible pathways for the photodegradation of propaphos in aqueous solution are shown in Scheme 1.

Toxicity evaluation

The acute oral LD₅₀ for rats is 70mg/kg. Inhalation LC50 (4 h) for rats is 39.2 mg/m3. NOEL (2yr) for rats is 0.08mg/kg/d. Propaphos orally administered to rats is rapidly excreted, mainly in the urine. The principal metabolic routes of propaphos are oxidation of the sulfide group to the sulfoxide and sulfone, and hydrolysis of phenyl phosphate ester bond in both animals and plants.