Flavin adenine dinucleotide--dependent monooxygenase: its role in the sulfoxidation of pesticides in mammals.

The flavin adenine dinucleotide--dependent monooxygenase in mammalian hepatic microsomes plays a major role in the oxidative metabolism of thioether-containing pesticides. Thirty-four compounds were tested, and it was determined that organophosphorus insecticides such as disulfoton and phorate are rapidly oxidized by the purified enzyme to their corresponding sulfoxides. The enzyme does not catalyze the oxidation of the thiophosphoryl and thiol sulfur atoms of these or other phosphorothioates and phosphorodithioates, or the oxidation of the sulfoxide to the sulfone. Carbamates aldicarb and Croneton are also oxidized, but at a lower rate.

Insecticidal benzoylphenyl ureas: structure-activity relationships as chitin synthesis inhibitors.

The 1-benzoyl-3-phenylurea insecticide diflubenzuron is a potent inhibitor for the conversion of (14)C-labeled glucose to (14)C-labeled chitin in isolated abdomens of newly emerged adult milkweed bugs (Oncopeltus fasciatus Dallas). The inhibitory activity of 24 diflubenzuron analogs in this in vitro chitin-synthesizing system is in good agreement with their toxicity to fifth instar nymphs of this species. These insecticides act quickly and directly within the integument to ultimately block the terminal polymerization step in chitin formation.

Sulfoxidation of thioether-containing pesticides by the flavin-adenine dinucleotide- dependent monooxygenase of pig liver microsomes.

In the presence of NADPH and under aerobic conditions, thioether-containing organo-phosphorus and carbamate pesticides were oxidized by the FAD-dependent monooxygenase (EC 1.14.13.8) purified from pig liver microsomes. The stoichiometric relationship between NADPH and substrate during the course of the reaction was 1:1, and the product, in the case of disulfoton and phorate, was the sulfoxide. The product was optically active and further oxidation to the sulfone was not apparent. Furthermore, the sulfoxides of disulfoton, phorate and croneton were not substrates for this enzyme. n-Octylamine, a known cytochrome P-450 inhibitor, increased the rate of sulfoxidation reactions catalyzed by the FAD-dependent monooxygenase. Structure-activity relationships were investigated using thirty-nine possible substrates. Structural changes around the thioether sulfur that affect nucleophilicity or that cause steric hindrance tended to decrease the sulfoxidation rats. With phosphorodithioates, changes around the phosphorus atom also affected oxidation of the thioether sulfur. Although neither the thiono nor the thiol sulfur atoms were attacked, substitution of either sulfur by oxygen decreased sulfoxidation. Thioether-containing O, O-dimethyl phosphorodithioates were not oxidized as readily as their O, O-diethyl analogs. Tetram and its analogs, which contain a teritiary amine group, were also substrates for this enzyme, presumably forming the N-oxide.

S-(O-ethyl phenylphosphonothionyl) glutathione; evidence for its formation in the in vitro metabolism of EPN in houseflies.

Double labelling experiments using [phenyl-14C], [2,6-p-nitro-phenyl-14C]- or, nonradioactive EPN with [35S]-, [glycine-3H]- or nonradioactive glutathione in the presence of the 100,000g supernatant fraction from the Rutgers strain of houseflies resulted in the formation of a radiolabelled conjugate. Hydrolysis of the [phenyl-14C] conjugate formed the phenyl-[14C]-phosphonothioic acid. The proposed structure of the conjugate is S-(O-ethyl phenylphosphonothionyl) glutathione. The conjugate is formed with racemic, (+), or (-) EPN. Structure activity studies indicate the following: a decrease in the electron withdrawing properties of the substituents on the O-p-nitrophenyl moiety results in a decrease in the formation of the conjugates; substituting the phenylphosphonothioic moiety with the ethyl-phosphonothioate, inhibits the formation of the corresponding conjugate' substituting the O-ethyl group with other alkyl groups did not effect the formation of the corresponding conjugate.