Environmental biotransformation mechanisms by flavin-dependent monooxygenase: A computational study.

The enzyme-catalyzed metabolic biotransformation of xenobiotics plays a significant role in toxicology evolution and subsequently environmental health risk assessment. Recent studies noted that the phase I human flavin-dependent monooxygenase (e.g., FMO3) can catalyze xenobiotics into more toxic metabolites. However, details of the metabolic mechanisms are insufficient. To fill the mechanism in the gaps, the systemic density functional theory calculations were performed to elucidate diverse FMO-catalyzed oxidation reactions toward environmental pollutants, including denitrification (e.g., nitrophenol), N-oxidation (e.g., nicotine), desulfurization (e.g., fonofos), and dehalogenation (e.g., pentachlorophenol). Similar to the active center compound 0 of cytochrome P450, FMO mainly catalyzed reactions with the structure of the tricyclic isoalloxazine C-4a-hydroperoxide (FADHOOH). As will be shown, FMO-catalyzed pathways are more favorable with a concerted than stepwise mechanism; Deprotonation is necessary to initiate the oxidation reactions for phenolic substrates; The regioselectivity of nicotine by FMO prefers the N-oxidation other than N-demethylation pathway; Formation of the P-S-O triangle ring is the key step for desulfurization of fonofos by FMO. We envision that these fundamental mechanisms catalyzed by FMO with a computational method can be extended to other xenobiotics of similar structures, which may aid the high-throughput screening and provide theoretical predictions in the future.

Identification of fonofos metabolites in Latuca sativa, Beta vulgaris, and Triticum aestivum by packed capillary flow fast atom bombardment tandem mass spectrometry.

The metabolism of fonofos, a thiophosphonate insecticide, was investigated in mature lettuce (Latuca sativa), beet (Beta vulgaris), and wheat (Triticum aestivum). Six new metabolites were identified by LC-MS and LC-MS-MS analysis using fast atom bombardment (FAB) and packed capillary LC columns with application of the on-column focusing technique. These methods provided the sensitivity required to identify unknown metabolites that were present in the mature plants at only 20-230 ppb. Structural elucidation was facilitated by use of fonofos labeled with both carbon-14 and carbon-13 in the phenyl ring. In all three plants fonofos was converted to a glucose conjugate of thiophenoxylactic acid. Oxidation of the glucose conjugate gave isomeric sulfoxides in all species examined. Thiophenoxylactic acid was found esterified to malonic acid in lettuce. In beets, S-phenylcysteine was found as its malonic acid amide. A second metabolite unique to beets was N-(malonyl)-[2[(ethoxyethylphosphinothionyl)oxy]phenyl]cysteine. This novel structure was confirmed by synthesis.

Docking study of enantiomeric fonofos oxon bound to the active site of Torpedo californica acetylcholinesterase.

Molecular interaction between enantiomeric fonofos oxon (O-ethyl S-phenyl ethylphosphonothiolate) and acetylcholinesterase (AChE) of Torpedo californica was evaluated by using the Cerius2 program. It was suggested that the difference in the inhibitory activity of the two enantiomers of fonofos oxon on AChE is due to the steric hindrance in binding to the AChE active site.

Fungal degradation of organophosphorus insecticides.

Organophosphorous insecticides are used extensively in agriculture. As a group, they are easily degraded by bacteria in the environment. However, a number of them have half-lives of several months. Little is known about their biodegradation by fungi. We showed that Phanerochaete chrysosporium mineralized chlorpyrifos, fonofos, and terbufos (27.5, 12.2, and 26.6%, respectively) during an 18-d incubation in nutrient nitrogen-limited cultures. Results demonstrated that the chlorinated pyridinyl ring of chlorpyrifos and the phenyl ring of fonofos undergo cleavage during biodegradation by the fungus. The usefulness of P. chrysosporium for bioremediation is discussed.

Absorption and metabolism of the chiral isomers of fonofos in the corn and cotton plant.

Root absorption of chiral phenyl-35S-fonofos in cotton and corn plants revealed stereoselective differences between the two enantiomers. (S)p-Fonofos was absorbed at a faster initial rate and to a greater extent than the (R)p enantiomer in both plant species. Approximately 40% and 62% of the applied radioactivity was absorbed into the cotton plant 12 hr after application of (R)p- and (S)p-fonofos, respectively. In the corn plant, approximately 25% and 63% of the applied (R)p- and (S)p-fonofos was absorbed in the first 12 hrs. Little qualitative or quantitative difference in plant translocation between fonofos enantiomers was observed. (R)p-fonofos was found to be metabolized to a greater extent than the (S)p enantiomer in both cotton and corn plants.

Environmental factors affecting the degradation of Dyfonate by soil fungi.

The ability of selected fungi to degrade the soil insecticide Dyfonate (O-ethyl S-phenyl ethylphosphonodithioate) into water-soluble, noninsecticidal metabolites was found to be dependent on the supply of nutrients, incubation time, temperature, pH, as well as other factors. With yeast extract as the carbon source (5 g/liter) and ammonium nitrate (1 g/liter) as the nitrogen source, both Rhizopus arrhizus and Penicillium notatum degraded the insecticide to a larger extent than with any other combination of nutrients used. With glucose as the carbon source, concentrations of ammonium nitrate above 5 g/liter inhibited the degradation of Dyfonate by R. arrhizus. Time-course studies on the metabolism of the insecticide indicated that Dyfonate was first absorbed by the fungal mycelium, where it was metabolized followed by the release of water-soluble, noninsecticidal, breakdown products into the culture media. The degradation appeared to involve the breakdown of Dyfonate into ethyl acetate soluble metabolites, such as ethylethoxyphosphonothioic acid, ethylethoxyphosphonic acid, methyl phenyl sulfoxide, and methyl phenyl sulfone. These compounds were then further degraded into water-soluble products. The optimum conditions for the degradation of the insecticide by R. arrhizus were observed at pH 6.0 to 7.0 and at 15-25 degrees C. Aged fungal mycelia were as active as mycelia in the logarithmic growth phase.