title: Synthesis of fenthion sulfoxide and fenoxon sulfoxide enantiomers: effect of sulfur chirality on acetylcholinesterase activity.
abstract: 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.
authors.affiliation: Department of Medicinal Chemistry and Environmental Toxicology Research Program, University of Mississippi, Mississippi 38677, USA.
authors.forename: Rama Sarma V S
authors.initials: RS
authors.lastname: Gadepalli
authors.forename: John M
authors.initials: JM
authors.lastname: Rimoldi
authors.forename: Frank R
authors.initials: FR
authors.lastname: Fronczek
authors.forename: Mae
authors.initials: M
authors.lastname: Nillos
authors.forename: Jay
authors.initials: J
authors.lastname: Gan
authors.forename: Xin
authors.initials: X
authors.lastname: Deng
authors.forename: Gabriela
authors.initials: G
authors.lastname: Rodriguez-Fuentes
authors.forename: Daniel
authors.initials: D
authors.lastname: Schlenk
mesh_terms: D000110:Acetylcholinesterase / Q000737:chemistry / Q000187:drug effects*; D018360:Crystallography, X-Ray; D004789:Enzyme Activation / Q000187:drug effects; D005284:Fenthion / Q000031:analogs & derivatives* / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology*; D008958:Models, Molecular; D015394:Molecular Structure; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013454:Sulfoxides / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology*
chemical_list: D013454:Sulfoxides; C519541:fenoxon sulfoxide; D005284:Fenthion; D000110:Acetylcholinesterase