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Katsuhiko Nakamuro - One of the best experts on this subject based on the ideXlab platform.
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Aqueous photodegradation of Fenthion by ultraviolet B irradiation: contribution of singlet oxygen in photodegradation and photochemical hydrolysis.
Water research, 2003Co-Authors: Yoshichika Hirahara, Hitoshi Ueno, Katsuhiko NakamuroAbstract:Abstract The objective of this study was to evaluate the photodegradation of the organophosphorus pesticide Fenthion in the environment from a human health effect viewpoint. The major photodegradation products of Fenthion in an aqueous solution under UVB irradiation (280–320 nm radiation) were identified as Fenthion sulfoxide, 3-methyl-4-methylthiophenol (MMTP), dimethyl phosphorothioate and 3-methyl-4-methylsulfinylphenol (MMSP). MMTP, dimethyl phosphorothioate and MMSP were discovered as novel photodegradation products of Fenthion. Kinetic analysis of these products showed the formation of MMTP and dimethyl phosphorothioate by the photochemical hydrolysis of Fenthion, which was accelerated under alkaline conditions. The former was further oxidized to MMSP. Fenthion sulfoxide was directly produced by the oxidative reaction of Fenthion. Contribution of dissolved oxygen in this photooxidation was observed by replacing the air with nitrogen gas in the reaction system, which prevented oxidative formation of Fenthion sulfoxide from Fenthion and MMSP from MMTP. These oxidative compounds were also formed from Fenthion in the presence of singlet oxygen ( 1 O 2 ) generated by the visible light irradiation of rose bengal solution, while 1 O 2 scavengers, l -histidine and sodium azide (NaN 3 ) inhibited this reaction. The aqueous photolysis mechanisms of Fenthion were proposed from a kinetic photolysis experiment study as follows: there were two kinds of UVB photodegradation pathways of Fenthion, one being photochemical hydrolysis of the phosphorus- O -phenyl ester to form MMTP and dimethyl phosphorothioate, and the other oxygenation triggered by 1 O 2 and producing Fenthion sulfoxide and MMSP. Therefore, the steady photodegradation products of Fenthion in the water environment may be Fenthion sulfoxide and MMSP.
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Photooxidation Mechanism of Fenthion by Singlet Oxygen: Evidence by ESR Analysis with a Selective Spin Trapping Agent
JOURNAL OF HEALTH SCIENCE, 2003Co-Authors: Yoshichika Hirahara, Hitoshi Ueno, Tomofumi Okuno, Katsuhiko NakamuroAbstract:Our previous study suggested that UVB irradiation of the organophosphorus pesticide, Fenthion and its photodegradation product, 3-methyl-4-methylthiophenol (MMTP) yielded Fenthion sulfoxide and 3-methyl-4methylsulfinylphenol (MMSP), and that the formation mechanism was related to generation of singlet oxygen ( 1 O2). The objective of this study was to elucidate the 1 O2-triggered photooxidation mechanism of Fenthion in detail. Generation of 1 O2 in the photooxidative reaction was directly detected by electron spin resonance (ESR) technique with 2,2,6,6-tetramethyl-4-piperidone (TMPD) as a selective 1 O2 spin trapping agent which yields 2,2,6,6-tetramethyl-4piperidone-1-oxyl (TEMPONE). When Fenthion and MMTP solutions were irradiated by UVA or UVB light, the TEMPONE signal was observed. However, no signal was detected after exposure of MMSP, dimethyl phosphorothioate or Fenthion sulfoxide solutions to the UV light. The production of the signal in Fenthion and MMTP solutions was more predominant under UVB irradiation than under UVA irradiation. When the signal was detected in these solutions, Fenthion sulfoxide and MMSP were also formed in amounts proportional to the signal intensity. The TEMPONE signal intensity and the formation of these oxidative products were significantly inhibited by the addition of 1 O2 scavenger, L-histidine or sodium azide to the reaction medium. The study provided the first direct evidence that 1 O2 is generated during photolysis of Fenthion and MMTP by UV irradiation. We also proposed its oxidation mechanism of Fenthion and MMTP.
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comparative photodegradation study of Fenthion and disulfoton under irradiation of different light sources in liquid and solid phases
Journal of Health Science, 2001Co-Authors: Yoshichika Hirahara, Hitoshi Ueno, Katsuhiko NakamuroAbstract:To assess the photodegradation extent of the organophosphorus pesticides, Fenthion and disulfoton in the environment, degradation rates of their chemicals under irradiation by visible light and each region of the ultraviolet (UV) sources, UVA (320‐400 nm radiation), UVB (280‐320 nm radiation) and UVC (250‐260 nm radiation) in liquid- and solid-phases were determined and their photolysis products identified. Although visible light did not cause photodegradation of either Fenthion or disulfoton even after the maximal irradiation of 8 hr in both phases, all the ranges of UV sources resulted in the photolysis of both pesticides, despite the solid-phase. The photolysis rates of Fenthion under UVB and UVC irradiation in both phases were higher than that under UVA irradiation and also exceeded all of the degradation rates of disulfoton. This confirms that Fenthion is more readily degraded than disulfoton in the UVB range. Fenthion sulfoxide and disulfoton sulfoxide were identified as oxidative photolysis products whose photolysis rates were considerably slower than parent compounds, and the oxidation reaction of both pesticides in the liquid-phase was superior to those on the solid-phase, indicating that dissolved oxygen in water is an important factor affecting the photolysis. These results suggest that Fenthion in the environment is more rapidly degraded by UVB irradiation in natural sunlight than disulfoton, and sulfoxides of both pesticides remain stable as a photolysis product in the environment.
Shigeru Ohta - One of the best experts on this subject based on the ideXlab platform.
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In Vitro Metabolism of Fenthion and Fenthion Sulfoxide by Liver Preparations of Sea Bream, Goldfish, and Rats
Drug metabolism and disposition: the biological fate of chemicals, 2003Co-Authors: Shigeyuki Kitamura, Tomoko Kadota, Mayumi Yoshida, Tomoharu Suzuki, Koji Ohashi, Shigeru OhtaAbstract:The in vitro metabolism of Fenthion and its sulfoxide (Fenthion sulfoxide) in sea bream (Pagrus major) and goldfish (Carassius auratus) was investigated and compared with that in rats. Fenthion was oxidized to Fenthion sulfoxide and the oxon derivative, but not to its sulfone, in the presence of NADPH by liver microsomes of sea bream, goldfish, and rats. These liver microsomal activities of the fish were lower than those of rats but were of the same order of magnitude. The NADPH-linked oxon- and sulfoxide-forming activities of liver microsomes of the fish and rats were inhibited by SKF 525-A, metyrapone, α-naphthoflavone, and carbon monoxide. The oxidizing activity to Fenthion sulfoxide was also inhibited by α-naphthylthiourea. Several cytochrome P450 isoforms and flavin-containing monooxygenase 1 exhibited these oxidase activities. Fenthion sulfoxide was reduced to Fenthion with liver cytosol of the fish and rats upon addition of 2-hydroxypyrimidine,N1-methylnicotinamide, or butyraldehyde, each of which is an electron donor of aldehyde oxidase, under anaerobic conditions. The activity was inhibited by menadione, β-estradiol, and chlorpromazine, which are inhibitors of aldehyde oxidase. The activities in the fish livers were similar to those of rat liver. Aldehyde oxidase purified from the livers of sea bream and rats exhibited the reducing activity. Thus, Fenthion and Fenthion sulfoxide are interconvertible in fish and rats through the activities of cytochrome P450, flavin-containing monooxygenase, and aldehyde oxidase.
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Antiandrogenic activity and metabolism of the organophosphorus pesticide Fenthion and related compounds.
Environmental health perspectives, 2003Co-Authors: Shigeyuki Kitamura, Shigeru Ohta, Tomoharu Suzuki, Nariaki FujimotoAbstract:We investigated the endocrine-disrupting actions of the organophosphorus pesticide Fenthion and related compounds and the influence of metabolic transformation on the activities of these compounds. Fenthion acted as an antagonist of the androgenic activity of dihydrotestosterone (10(-7)M) in the concentration range of 10(-6)-10(-4)M in an androgen-responsive element-luciferase reporter-responsive assay using NIH3T3 cells. The antiandrogenic activity of Fenthion was similar in magnitude to that of flutamide. Fenthion also tested positive in the Hershberger assay using castrated male rats. Marked estrogenic and antiestrogenic activities of Fenthion and related compounds were not observed in MCF-7 cells. When Fenthion was incubated with rat liver microsomes in the presence of NADPH, the antiandrogenic activity markedly decreased, and Fenthion sulfoxide was detected as a major metabolite. The oxidase activity toward Fenthion was exhibited by cytochrome P450 and flavin-containing monooxygenase. Fenthion sulfoxide was negative in the screening test for antiandrogens, as was Fenthion sulfone. However, when Fenthion sulfoxide was incubated with liver cytosol in the presence of 2-hydroxypyrimidine, an electron donor of aldehyde oxidase, the extract of the incubation mixture exhibited antiandrogenic activity. In this case, Fenthion was detected as a major metabolite of the sulfoxide. Metabolic interconversion between Fenthion and Fenthion sulfoxide in the body seems to maintain the antiandrogenic activity.
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Whole-body metabolism of the organophosphorus pesticide, Fenthion, in goldfish, Carassius auratus
Comparative biochemistry and physiology. Toxicology & pharmacology : CBP, 2000Co-Authors: Shigeyuki Kitamura, Tomoko Kadota, Mayumi Yoshida, Norimasa Jinno, Shigeru OhtaAbstract:The in vivo metabolism of Fenthion, an organophosphorus pesticide, and its sulfoxide (Fenthion sulfoxide) was examined in goldfish (Carassius auratus). When goldfish were administered Fenthion i.p. at a dose of 100 mg/kg, two metabolites were isolated from the tank water. They were identified as Fenthion sulfoxide and Fenthion oxon, in which > P = S of Fenthion is transformed to > P = O, by comparing their mass and UV spectra, and their behavior in HPLC and TLC, with those of authentic standards. However, Fenthion sulfone was not detected as a metabolite. The amounts of Fenthion, Fenthion sulfoxide and Fenthion oxon excreted within 4 days were 2.7, 3.4 and 2.5%, of the initial dose of Fenthion, respectively. Unchanged Fenthion was detected in the body of the fish to the extent of 42-50% of the dose after 10 days, but Fenthion sulfoxide and Fenthion oxon showed very low concentrations. When Fenthion sulfoxide was administered to the fish, about 70% of the dose was excreted unchanged into the tank water within 24 h, but little of the reduced compound, Fenthion, was found. In contrast, Fenthion was detected at 2.1% of dose in the body of goldfish as a metabolite of Fenthion sulfoxide. The fact that Fenthion is metabolized to the toxic oxon form in fish presumably has environmental and health implication for its use as a pesticide.
Daniel Schlenk - One of the best experts on this subject based on the ideXlab platform.
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Mechanisms of Fenthion activation in rainbow trout (Oncorhynchus mykiss) acclimated to hypersaline environments
Toxicology and applied pharmacology, 2008Co-Authors: Ramon Lavado, John M. Rimoldi, Daniel SchlenkAbstract:Previous studies in rainbow trout have shown that acclimation to hypersaline environments enhances the toxicity to thioether organophosphate and carbamate pesticides. In order to determine the role of biotransformation in this process, the metabolism of the thioether organophosphate biocide, Fenthion was evaluated in microsomes from gills, liver and olfactory tissues in rainbow trout (Oncorhynchus mykiss) acclimated to freshwater and 17‰ salinity. Hypersalinity acclimation increased the formation of fenoxon and fenoxon sulfoxide from Fenthion in liver microsomes from rainbow trout, but not in gills or in olfactory tissues. NADPH-dependent and independent hydrolysis was observed in all tissues, but only NADPH-dependent Fenthion cleavage was differentially modulated by hypersalinity in liver (inhibited) and gills (induced). Enantiomers of Fenthion sulfoxide (65% and 35% R- and S-Fenthion sulfoxide, respectively) were formed in liver and gills. The predominant pathway of Fenthion activation in freshwater appears to be initiated through initial formation of fenoxon which may be subsequently converted to the most toxic metabolite fenoxon R-sulfoxide. However, in hypersaline conditions both fenoxon and Fenthion sulfoxide formation may precede fenoxon sulfoxide formation. Stereochemical evaluation of sulfoxide formation, cytochrome P450 inhibition studies with ketoconazole and immunoblots indicated that CYP3A27 was primarily involved in the enhancement of Fenthion activation in hypersaline-acclimated fish with limited contribution of FMO to initial sulfoxidation.
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Synthesis of Fenthion sulfoxide and fenoxon sulfoxide enantiomers: effect of sulfur chirality on acetylcholinesterase activity.
Chemical research in toxicology, 2007Co-Authors: Rama S.v. Gadepalli, John M. Rimoldi, Frank R. Fronczek, Mae Grace Nillos, Jay Gan, Xin Deng, Gabriela Rodríguez-fuentes, Daniel SchlenkAbstract:Earlier reports have demonstrated that recombinant flavin-containing monooxygenase 1 (FMO1) catalyzes the oxidation of the organophosphate pesticide Fenthion to (+)-Fenthion sulfoxide in a stereoselective fashion. In order to elucidate the absolute configuration of the sulfoxide metabolite produced, we established an efficient synthesis of both enantiomers of Fenthion sulfoxide, which were transformed into chiral fenoxon sulfoxides using a two-step protocol. The use of chiral oxidants, namely, N-(phenylsulfonyl)(3,3-dichlorocamphoryl) oxaziridines, afforded enantioenriched Fenthion sulfoxides with high ee (>82%) from the parent sulfide. Single recrystallizations afforded chiral Fenthion sulfoxides with >99% ee, measured by chiral HPLC analysis. The absolute configuration of the (+)-sulfoxide generated from Fenthion metabolism by FMO1 was determined to be (R)-(+)-Fenthion sulfoxide, confirmed by X-ray crystallographic analysis of the (S)-(-)-antipode. Inhibition of human recombinant (hrAChE) and electric eel (eeAChE) acetylcholinesterase were assayed with Fenthion, fenoxon, and the racemates and enantiomers of Fenthion sulfoxide and fenoxon sulfoxide. Results revealed stereoselective inhibition with (R)-(+)-fenoxon sulfoxide when compared with that of (S)-(-)-fenoxon sulfoxide (IC50 of 6.9 and 6.5 microM vs 230 and 111 microM in hrAChE and eeAChE, respectively). Fenthion sulfoxide (R or S enantiomers) did not present anti-AChE properties. Although the stereoselective sulfoxidation of Fenthion to (R)-(+)-Fenthion sulfoxide by FMO represents a detoxification pathway, the results of this study support the notion that subsequent oxidative desulfuration of (R)-(+)-Fenthion sulfoxide (in vivo) may represent a critical bioactivation pathway, resulting in the production of (R)-(+)-fenoxon sulfoxide, a potent AChE inhibitor.
Shigeyuki Kitamura - One of the best experts on this subject based on the ideXlab platform.
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In Vitro Metabolism of Fenthion and Fenthion Sulfoxide by Liver Preparations of Sea Bream, Goldfish, and Rats
Drug metabolism and disposition: the biological fate of chemicals, 2003Co-Authors: Shigeyuki Kitamura, Tomoko Kadota, Mayumi Yoshida, Tomoharu Suzuki, Koji Ohashi, Shigeru OhtaAbstract:The in vitro metabolism of Fenthion and its sulfoxide (Fenthion sulfoxide) in sea bream (Pagrus major) and goldfish (Carassius auratus) was investigated and compared with that in rats. Fenthion was oxidized to Fenthion sulfoxide and the oxon derivative, but not to its sulfone, in the presence of NADPH by liver microsomes of sea bream, goldfish, and rats. These liver microsomal activities of the fish were lower than those of rats but were of the same order of magnitude. The NADPH-linked oxon- and sulfoxide-forming activities of liver microsomes of the fish and rats were inhibited by SKF 525-A, metyrapone, α-naphthoflavone, and carbon monoxide. The oxidizing activity to Fenthion sulfoxide was also inhibited by α-naphthylthiourea. Several cytochrome P450 isoforms and flavin-containing monooxygenase 1 exhibited these oxidase activities. Fenthion sulfoxide was reduced to Fenthion with liver cytosol of the fish and rats upon addition of 2-hydroxypyrimidine,N1-methylnicotinamide, or butyraldehyde, each of which is an electron donor of aldehyde oxidase, under anaerobic conditions. The activity was inhibited by menadione, β-estradiol, and chlorpromazine, which are inhibitors of aldehyde oxidase. The activities in the fish livers were similar to those of rat liver. Aldehyde oxidase purified from the livers of sea bream and rats exhibited the reducing activity. Thus, Fenthion and Fenthion sulfoxide are interconvertible in fish and rats through the activities of cytochrome P450, flavin-containing monooxygenase, and aldehyde oxidase.
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Antiandrogenic activity and metabolism of the organophosphorus pesticide Fenthion and related compounds.
Environmental health perspectives, 2003Co-Authors: Shigeyuki Kitamura, Shigeru Ohta, Tomoharu Suzuki, Nariaki FujimotoAbstract:We investigated the endocrine-disrupting actions of the organophosphorus pesticide Fenthion and related compounds and the influence of metabolic transformation on the activities of these compounds. Fenthion acted as an antagonist of the androgenic activity of dihydrotestosterone (10(-7)M) in the concentration range of 10(-6)-10(-4)M in an androgen-responsive element-luciferase reporter-responsive assay using NIH3T3 cells. The antiandrogenic activity of Fenthion was similar in magnitude to that of flutamide. Fenthion also tested positive in the Hershberger assay using castrated male rats. Marked estrogenic and antiestrogenic activities of Fenthion and related compounds were not observed in MCF-7 cells. When Fenthion was incubated with rat liver microsomes in the presence of NADPH, the antiandrogenic activity markedly decreased, and Fenthion sulfoxide was detected as a major metabolite. The oxidase activity toward Fenthion was exhibited by cytochrome P450 and flavin-containing monooxygenase. Fenthion sulfoxide was negative in the screening test for antiandrogens, as was Fenthion sulfone. However, when Fenthion sulfoxide was incubated with liver cytosol in the presence of 2-hydroxypyrimidine, an electron donor of aldehyde oxidase, the extract of the incubation mixture exhibited antiandrogenic activity. In this case, Fenthion was detected as a major metabolite of the sulfoxide. Metabolic interconversion between Fenthion and Fenthion sulfoxide in the body seems to maintain the antiandrogenic activity.
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Whole-body metabolism of the organophosphorus pesticide, Fenthion, in goldfish, Carassius auratus
Comparative biochemistry and physiology. Toxicology & pharmacology : CBP, 2000Co-Authors: Shigeyuki Kitamura, Tomoko Kadota, Mayumi Yoshida, Norimasa Jinno, Shigeru OhtaAbstract:The in vivo metabolism of Fenthion, an organophosphorus pesticide, and its sulfoxide (Fenthion sulfoxide) was examined in goldfish (Carassius auratus). When goldfish were administered Fenthion i.p. at a dose of 100 mg/kg, two metabolites were isolated from the tank water. They were identified as Fenthion sulfoxide and Fenthion oxon, in which > P = S of Fenthion is transformed to > P = O, by comparing their mass and UV spectra, and their behavior in HPLC and TLC, with those of authentic standards. However, Fenthion sulfone was not detected as a metabolite. The amounts of Fenthion, Fenthion sulfoxide and Fenthion oxon excreted within 4 days were 2.7, 3.4 and 2.5%, of the initial dose of Fenthion, respectively. Unchanged Fenthion was detected in the body of the fish to the extent of 42-50% of the dose after 10 days, but Fenthion sulfoxide and Fenthion oxon showed very low concentrations. When Fenthion sulfoxide was administered to the fish, about 70% of the dose was excreted unchanged into the tank water within 24 h, but little of the reduced compound, Fenthion, was found. In contrast, Fenthion was detected at 2.1% of dose in the body of goldfish as a metabolite of Fenthion sulfoxide. The fact that Fenthion is metabolized to the toxic oxon form in fish presumably has environmental and health implication for its use as a pesticide.
Yoshichika Hirahara - One of the best experts on this subject based on the ideXlab platform.
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Aqueous photodegradation of Fenthion by ultraviolet B irradiation: contribution of singlet oxygen in photodegradation and photochemical hydrolysis.
Water research, 2003Co-Authors: Yoshichika Hirahara, Hitoshi Ueno, Katsuhiko NakamuroAbstract:Abstract The objective of this study was to evaluate the photodegradation of the organophosphorus pesticide Fenthion in the environment from a human health effect viewpoint. The major photodegradation products of Fenthion in an aqueous solution under UVB irradiation (280–320 nm radiation) were identified as Fenthion sulfoxide, 3-methyl-4-methylthiophenol (MMTP), dimethyl phosphorothioate and 3-methyl-4-methylsulfinylphenol (MMSP). MMTP, dimethyl phosphorothioate and MMSP were discovered as novel photodegradation products of Fenthion. Kinetic analysis of these products showed the formation of MMTP and dimethyl phosphorothioate by the photochemical hydrolysis of Fenthion, which was accelerated under alkaline conditions. The former was further oxidized to MMSP. Fenthion sulfoxide was directly produced by the oxidative reaction of Fenthion. Contribution of dissolved oxygen in this photooxidation was observed by replacing the air with nitrogen gas in the reaction system, which prevented oxidative formation of Fenthion sulfoxide from Fenthion and MMSP from MMTP. These oxidative compounds were also formed from Fenthion in the presence of singlet oxygen ( 1 O 2 ) generated by the visible light irradiation of rose bengal solution, while 1 O 2 scavengers, l -histidine and sodium azide (NaN 3 ) inhibited this reaction. The aqueous photolysis mechanisms of Fenthion were proposed from a kinetic photolysis experiment study as follows: there were two kinds of UVB photodegradation pathways of Fenthion, one being photochemical hydrolysis of the phosphorus- O -phenyl ester to form MMTP and dimethyl phosphorothioate, and the other oxygenation triggered by 1 O 2 and producing Fenthion sulfoxide and MMSP. Therefore, the steady photodegradation products of Fenthion in the water environment may be Fenthion sulfoxide and MMSP.
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Photooxidation Mechanism of Fenthion by Singlet Oxygen: Evidence by ESR Analysis with a Selective Spin Trapping Agent
JOURNAL OF HEALTH SCIENCE, 2003Co-Authors: Yoshichika Hirahara, Hitoshi Ueno, Tomofumi Okuno, Katsuhiko NakamuroAbstract:Our previous study suggested that UVB irradiation of the organophosphorus pesticide, Fenthion and its photodegradation product, 3-methyl-4-methylthiophenol (MMTP) yielded Fenthion sulfoxide and 3-methyl-4methylsulfinylphenol (MMSP), and that the formation mechanism was related to generation of singlet oxygen ( 1 O2). The objective of this study was to elucidate the 1 O2-triggered photooxidation mechanism of Fenthion in detail. Generation of 1 O2 in the photooxidative reaction was directly detected by electron spin resonance (ESR) technique with 2,2,6,6-tetramethyl-4-piperidone (TMPD) as a selective 1 O2 spin trapping agent which yields 2,2,6,6-tetramethyl-4piperidone-1-oxyl (TEMPONE). When Fenthion and MMTP solutions were irradiated by UVA or UVB light, the TEMPONE signal was observed. However, no signal was detected after exposure of MMSP, dimethyl phosphorothioate or Fenthion sulfoxide solutions to the UV light. The production of the signal in Fenthion and MMTP solutions was more predominant under UVB irradiation than under UVA irradiation. When the signal was detected in these solutions, Fenthion sulfoxide and MMSP were also formed in amounts proportional to the signal intensity. The TEMPONE signal intensity and the formation of these oxidative products were significantly inhibited by the addition of 1 O2 scavenger, L-histidine or sodium azide to the reaction medium. The study provided the first direct evidence that 1 O2 is generated during photolysis of Fenthion and MMTP by UV irradiation. We also proposed its oxidation mechanism of Fenthion and MMTP.
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comparative photodegradation study of Fenthion and disulfoton under irradiation of different light sources in liquid and solid phases
Journal of Health Science, 2001Co-Authors: Yoshichika Hirahara, Hitoshi Ueno, Katsuhiko NakamuroAbstract:To assess the photodegradation extent of the organophosphorus pesticides, Fenthion and disulfoton in the environment, degradation rates of their chemicals under irradiation by visible light and each region of the ultraviolet (UV) sources, UVA (320‐400 nm radiation), UVB (280‐320 nm radiation) and UVC (250‐260 nm radiation) in liquid- and solid-phases were determined and their photolysis products identified. Although visible light did not cause photodegradation of either Fenthion or disulfoton even after the maximal irradiation of 8 hr in both phases, all the ranges of UV sources resulted in the photolysis of both pesticides, despite the solid-phase. The photolysis rates of Fenthion under UVB and UVC irradiation in both phases were higher than that under UVA irradiation and also exceeded all of the degradation rates of disulfoton. This confirms that Fenthion is more readily degraded than disulfoton in the UVB range. Fenthion sulfoxide and disulfoton sulfoxide were identified as oxidative photolysis products whose photolysis rates were considerably slower than parent compounds, and the oxidation reaction of both pesticides in the liquid-phase was superior to those on the solid-phase, indicating that dissolved oxygen in water is an important factor affecting the photolysis. These results suggest that Fenthion in the environment is more rapidly degraded by UVB irradiation in natural sunlight than disulfoton, and sulfoxides of both pesticides remain stable as a photolysis product in the environment.