OXIDATIVE BIOACTIVATION OF METHAMIDOPHOS INSECTICIDE - SYNTHESIS OF N-HYDROXYMETHAMIDOPHOS (A CANDIDATE METABOLITE) AND ITS PROPOSED ALTERNATIVE REACTIONS INVOLVING N-]O REARRANGEMENT OR FRAGMENTATION THROUGH A METAPHOSPHATE ANALOG

The systemic insecticide methamidophos, MeO(MeS)P(O)NH2, is a very wea k inhibitor of acetylcholinesterase (AChE) in vitro relative to in viv o suggesting bioactivation. This hypothesis is supported by finding th at brain AChE inhibition and poisoning signs from methamidophos are gr eatly delayed in mice and houseflies pretreated with oxidase inhibitor s in an order for effectiveness of methimazole > N-benzylimidazole >> piperonyl butoxide. In contrast, the order for delaying parathion-indu ced AChE inhibition and toxicity is N-benzylimidazole >> piperonyl but oxide or methimazole, suggesting that different oxidases are involved in methamidophos and parathion activation. N-Hydroxylation is examined here as an alternative to the controversial S-oxidation proposed earl ier for methamidophos activation. N-Hydroxymethamidophos [MeO(MeS)P(O) NHOH], synthesized by coupling MeO(MeS)P(O)Cl and Me-3-SiNHOSiMe3 foll owed by desilylation, is unstable at pH 7.4 (t(1/2) = 10 min at 37 deg rees C) with decomposition by two distinct and novel mechanisms. The f irst mechanism (A) is N --> O rearrangement to MeO(MeS)P(O)ONH2 and th en hydrolysis to MeO(MeS)P(O)OH, a sequence also established in the an alogous series of (MeO)(2)P(O)NHOH --> (MeO)(2)P(O)ONH2 --> (MeO)(2)P( O)OH. The second mechanism (B) is proposed to involve tautomerism to t he phosphimino form [MeO(MeS)P(OH)=NOH] that eliminates MeSH forming a metaphosphate analogue [MeOP(O)=NOH] trapped by water to give MeO(HO) P(O)NHOH that undergoes the N --> O rearrangement as above and hydroly sis to MeOP(O)(OH)(2). As a metaphosphate analogue, the metaphosphorim idate generated from MeO(MeS)P(O)NHOH in aqueous ethanol yields MeOP(O )(OH)(2) and MeO(EtO)P(O)OH in the same ratio as the solvents on a mol ar basis. Reactions of the N- and O-methyl derivatives of MeO(MeS)P(O) NHOH and (MeO)(2)P(O)NHOH are consistent with proposed mechanisms A an d B. N-Hydroxymethamidophos is less potent than methamidophos as an AC hE inhibitor and toxicant possibly associated with its rapid hydrolysi s. Bioactivation of methamidophos via a metaphosphate analogue would d irectly yield a phosphorylated and aged AChE resistant to reactivating agents, an intriguing hypothesis worthy of further consideration.