Isofenphos [O-ethyl O-[2-(isopropoxycarbonyl)phenyl] isopropylphosphoramidothionate] is bioactivated by mixed-function oxidases (MFOs) in two steps that ultimately give N-desisopropylisofenphos oxon, a product with 2300-fold greater inhibitory potency than isofenphos oxon toward house fly head acetylcholinesterase (AChE). Bioactivation of N-alkyl analogues of isofenphos oxon depends on an N-alkyl a-carbon proton and the steric bulk of the largest a-carbon substituent as described by Hansch regression analysis. These reactions are largely mediated by cytochrome P-450 and probably involve N-dealkylation via a-carbon oxidation without formation of AChE-inhibitory intermediates. Thiono oxidation is also required for the phosphorothionates, but MFO inhibition accompanying desulfuration slows the overall bioactivation. Findings on house fly toxicities are generally consistent with the in vitro bioactivation models. The isofenphos isomers, resolved on a chiral HPLC column, have the same stereospecificity for MFO-activated anti-AChE activity and toxicity to house flies, possibly due to enantiomeric differences in AChE inhibitory potency.
All Science Journal Classification (ASJC) codes
- Agricultural and Biological Sciences(all)