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 CO
2. 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 LD
50
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.
