Diethyl paraoxon

Recently, Noort et al developed a procedure that is based on straightforward isolation of adducted BuChE from plasma by means of affinity chromatography with a procainamide column, followed by pepsin digestion and LC/electrospray tandem MS analysis of a specific nonapeptide containing the phosphonylated active site serine-198 residue (5). This method surpasses the limitations of the fluoride-reactivation method, since it can also deal with dealkylated ( aged ) phosphonylated BuChE. The method allowed the positive analysis of several serum samples of Japanese victims of the terrorist attack in the Tokyo subway in 1995. Furthermore, the method could be applied for detection of ChE modifications induced by, e.g., diethyl paraoxon and pyridostigmine bromide, illustrating the broad scope of this approach. This new approach... 

The rat LD qS are 13, 3.6 (oral) and 21, 6.8 (dermal) mg/kg. Parathion is resistant to aqueous hydrolysis, but is hydroly2ed by alkah to form the noninsecticidal diethjlphosphorothioic acid and -nitrophenol. The time required for 50% hydrolysis is 120 d ia a saturated aqueous solution, or 8 h ia a solution of lime water. At temperatures above 130°C, parathion slowly isomerizes to 0,%diethyl 0-(4-nitrophenyl) phosphorothioate [597-88-6] which is much less stable and less effective as an insecticide. Parathion is readily reduced, eg, by bacillus subtilis ia polluted water and ia the mammalian mmen to nontoxic 0,0-diethyl 0-(4-aminophenyl) phosphorothioate, and is oxidized with difficulty to the highly toxic paraoxon [511-45-5] diethyl 4-nitrophenyl phosphate d 1.268, soluble ia water to 2.4 mg/L), rat oral LD q 1.2 mg/kg. 

Acetaminophen, which depletes hepatic glutathione, does not potentiate the toxicity of methyl parathion in mice. A possible mechanism of action may be competition between acetaminophen and methyl parathion for mixed function oxidases and subsequent prevention of activation of methyl parathion to methyl paraoxon (Costa and Murphy 1984). Diethyl maleate, an agent that depletes cytosolic glutathione and is not an enzyme inducer, potentiates toxicity of methyl parathion in mice (Mirer et al. 1977). 

Sultatos LG, Huang GJ, Jackson O, et al. 1991. The effect of glutathione monoethyl ester on the potentiation of the acute toxicity of methyl parathion, methyl paraoxon or fenitrothion by diethyl maleate in the mouse. Toxicol Lett 55 77-83. 

Fits for the La3 +-catalyzed methanolysis of /xnitrophenyl acetate (2),7 paraoxon (1), its thio derivative, 0,0-Diethyl. S -p-nitrophcnyl phosphorothioate (3)10b and IV-p-nitrophenyl /1-lactam (4)32 have also been computed, with the various ky" the values being given in Table 4. 

The following metabolites were identified in a soil-oat system paraoxon, aminoparathion, 4-nitrophenol, and 4-aminophenol (Lichtenstein, 1980 Lichtenstein et al., 1982). Mick and Dahm (1970) reported that Rhizobium sp. converted 85% [ CJparathion to aminoparathion and 10% diethyl phosphorothioic acid in 1 d. 

Parathion degraded on both glass surfaces and on bean plant leaves. Metabolites reported were paraoxon, 4-nitrophenol, and a compound tentatively identified as 5-ethyl parathion (El-Refai and Hopkins, 1966). Upon exposure to high intensity UV light, parathion was altered to the following photoproducts paraoxon, 0,5-diethyl 0-4-nitrophenyl phosphorothioate, 0,0-diethyl 5-4-nitrophenyl phosphorothioate, 0,0-bis(4-nitrophenyl) 0-ethyl phosphorothioate, 0,0-bis(4-nitrophenyl) 0-ethyl phosphate, 0,0-diethyl 0-phenyl phosphorothioate, and 0,0-diethyl 0-phenyl phosphate (Joiner et al., 1971). 

PB was reacted with the following model compounds paraoxon (diethyl j-nltrophenyl phosphate), EPMP (ethyl Tiiltrophenyl methylphosphonate), PDEP (jg -nltrophenyl dlethylphosphlnate, PNPA ( P7nltrophenyl acetate), and PMP (hydrogen nltrophenyl methylphosphonate). 

The majority of the diethyl phosphate and -nitrophenol formed in the mammalian metabolism of parathion is undoubtedly derived by the action of esterases or phosphatases ( ) on paraoxon formed from parathion in a cytochrome P-450-catalyzed reaction. However, a significant portion of the diethyl phosphate and -nitrophenol must also be the result of the attack of water on the intermediate S-oxide of parathion ( ). 

It Is well known that phosphorothlonate insecticides such as parathlon (, 0-diethyl p-nitrophenyl phosphorothloate) and malathion [0, -dimethyl -(l,2 -dlcarbethoxy)ethyl phosphoro-dithioate] are Intrinsically poor inhibitors of acetylcholinesterase and in vivo activation to the respective anticholinesterases paraoxon and malaoxon is required before animals exposed to the phosphorothionates are intoxicated. Since metabolic activation is essential to the biological activity of these thiono sulfur-containing organophosphorus insecticides, compounds of this type may be considered as propesticides or, more specifically, prolnsectlcldes. 

Mills and Hoffmann (1992) investigated ultrasonic degradation of parathion. Parathion (0,0-diethyl O-p-nitrophenyl triphosphate) is a major pesticide used in large quantities worldwide. Organophosphate esters such as parathion have been used as alternatives to DDT and other chlorinated hydrocarbon pesticides however, the organophosphate esters are not rapidly degraded in natural waters. At 20°C and pH 7.4, parathion has a hydrolytic half-life of 108 days and its toxic metabolite, paraoxon, has a similar half-life of 144 days. Ultrasonic irradiation of 25 mL of parathion-saturated, deionized water solution was conducted in a water-jacketed, stainless-steel cell with a Branson 200 sonifier operating at 20 kHz and 75 W/cm2. The temperature of the sonicated solution was kept constant at 30°C. All sonolytic reactions were carried out in air-saturated solution. The concentration of the parathion hydrolysis product p-nitrophenol (PNP) was determined in alkaline solution with a Shimadzu MPS-2000 UV /visible spectrophotometer. 

Sodium phosphate monobasic [S 9638], sodium phosphate dibasic [S 0876], sodium chloride [S 7653], acetylcholinesterase from Elect-rophorus electricus (Type V-S) [C 2888], potassium chloride [P 3911], 1,2-diaminobenzenedihydrochloride [P 1526], paraoxon (o,o-diethyl o-4-nitrophenyl phosphate) [D 9286], ferrocene carboxylic acid [106887], aniline [A 9880] and acetylthiocholine chloride [A 5751] were purchased from the Sigma Chemical Company (Dorset, UK). Screen-printed transducers were purchased from Gwent Electronic Materials Ltd. (Gwent, Wales, UK). These electrode assemblies comprised a working electrode based on carbon ink doped with cobalt phthalocya-nine, an on board reference electrode (Ag/AgCl) and counter electrode (platinum) (see Fig. 24.1). 

The methanolyses of a series of <9,0-diethyl 0-aryl phosphates (101 X = O) and 0,0-diethyl 5-aryl phosphorothioates (101 X = S) promoted by methoxide and two metal ion systems, (La3+)2(MeO )2 and a complex of Zn2+ (MeO-) with 1,5,9-triazacyclododecane (79 M = Zn), has been studied in methanol at 25 °C. The kinetic data for the metal-catalysed reactions were analysed in terms of a common mechanism where there is extensive cleavage of the P-XAr bond in the transition state. The relevance of these findings to the mechanism of action of a phosphotriesterase enzyme present in a soil bacterium that hydrolyses paraoxon was discussed.100... 

The methanolyses of several phosphate/phosphonate esters and their thio analogues [e.g. 0,0-diethyl 0-(4-nitrophenyl) phosphate, paraoxon (101 X = O, Z = 4-N02), 0,0-diethyl S- S-dichlorophenyl) phosphorothioate, (102 R = OEt), and 0-ethyl S-OA-dichlorophenyl) methylphosphonothioate (102 R = Me)] catalysed by methoxide and the complex of Zn2+ (MeO-) with 1,5,9-triazacyclododecane (79 M = Zn) were studied in methanol at 25 °C. The reaction of methoxide and (79 M = Zn) with the entire series of esters appears to adhere to a common mechanism that involves pre-equilibrium binding of the substrate, followed by intramolecular attack of the coordinated methoxide concerted with OAr or SAr leaving group departure.79... 

Disposition in the Body. Parathion is activated in the liver by metabolism to paraoxon. Parathion and paraoxon are further metabolised to diethylthiophosphoric acid (DETP), diethyl-phosphoric acid (DEP), and 4-nitrophenol which are the major urinary excretion products although DETP and DEP are unstable in stored urine. Urinary 4-nitrophenol concentrations may be indicative of the extent of exposure to parathion. 4-Nitrophenol is rapidly excreted in the urine and is not detectable 48 hours after exposure by inhalation or ingestion, but excretion is more prolonged after exposure of intact skin due to the much slower absorption of parathion by this route. Aminoparathion has been detected in postmortem blood and tissues. 

NTE was first identified as the presumptive target of neuropathic OP compounds in the initiation of OPIDN (Johnson, 1970). Its activity in cells and tissues is operationally defined as the enzymatic hydrolysis of the non-physiological substrate, phenyl valerate, which is resistant to inhibition by diethyl 4-nitrophenyl phosphate (paraoxon) and sensitive to inhibition by A.A -diisopropylphosphor-odiamidic fluoride (mipafox) under specified conditions of preincubation with inhibitors and subsequent incubation with substrate (Johnson, 1977 Kayyali et al., 1991 Makhaeva et al., 2007). 

Figure 3 Kinetic mechanism for the interaction of paraoxon (S) with A-esterase (E) or acetylcholinesterase (E). p-Nitrophenol (P,) is the first product released, whereas diethyl phosphate (Pa) is the second.

As outlined in Figure 3, the hydrolysis of paraoxon by human serum A-esterase(s) is very similar to the phosphorylation of B-esterases, such as acetylcholinesterase, by paraoxon. Both reactions involve an initial binding of paraoxon to the enzyme, followed by a rapid conformational change that produces diethyl phosphate and p-nitrophenol from paraoxon. p-Nitrophenol is quickly released from the enzyme, leaving diethyl phosphate covalently bound to enzyme. At this point, A-esterase quickly releases diethyl phosphate as a result of interacting with a water molecule. However, B-esterases, such as acetylcholinesterase, retain the diethyl phosphate for a much longer period of time, thereby resulting in inhibition of the enzyme. 

The residue Asp-301, which is hydrogen bonded to His-254, has been proposed to deprotonate the bridging hydroxide nucleophile.253 Mutation of His-245 to Ala or Asp reduces the rate of hydrolysis of paraoxon by 1-2 orders of magnitude. In contrast, the hydrolysis for the slower substrate diethyl p-chlorophenyl phosphate increases by from 2- to 33-fold.253 This suggests that for the more activated... 

Structures of several common substrates and an inhibitor used to study the mechanisms of OPA anhydrase activity. DFP (diisopropylfluorophosphate), mipafox (N,N -diisopropylphosphorodi-amidofluoridate), tabun (N,N-dimethylethylphosphoroamidocyanidate), soman(0-l,2,2-trimethylpropylmethylphosphonofluoridate, paraoxon (diethyl 4-nitrophenyl phosphate), and parathion. 

Horse serum 7.5, 9.5 Diethyl 4-nitrophenyl phosphate (Paraoxon) and l-chloro-l-ethylcarbamoyl-l-propen-2-yl-dimethyl phosphate (Phosphamidon) W4... 

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