Organophosphorus inhibitors of insect juvenile hormone esterase

Three classes of organophosphorus inhibitors, O-alkyl ethylphosphonates, enol phosphates, and α-thioalkylphosphonates have been prepared and assayed as inhibitors of insect juvenile hormone (JH) esterase. The O-alkyl ethylphosphonates were prepared from ethylphosphonic dichloride by sequential addition of sodium 4-nitrophenoxide and the appropriate lithium alkoxide. The enol phosphates were prepared via the Perkow reaction from α-chloro aldehydes. Ethyl O-4-nitrophenyl octylthiomethylphosphonate (ENOSP) and ethyl S-phenyl octylthiomethylphosphonothioate (EPOSP) were prepared from diethyl thiomethylphosphonate. The O-alkyl phosphonate series exhibited in vitro activity ( I 50 = 10 −6 to 10 −8 M) as inhibitors for JH esterase. In in vivo studies, n-hexyl O-4-nitrophenyl ethylphosphonate (ONEP) was a persistent but nonselective inhibitor of JH esterase. The enol phosphates were ineffective JH esterase inhibitors in in vitro and in vivo studies. The α-thioalkylphosphonates ENOSP and EPOSP were very potent JH esterase inhibitors, I 50 = 1.0 × 10 −9 and 4.9 × 10 −9 M, respectively. Both ENOSP and EPOSP were also persistent in vivo inhibitors which exhibited considerable selectivity for JH esterase over α-naphthyl acetate esterase. A kinetic analysis of ENOSP determined that inhibition followed pseudo-first-order kinetics in which a reversible enzyme/inhibitor complex precedes irreversible phosphorylation. The bimolecular rate constant ( k i) was 6.52 × 10 7 M −1 min −1, the phosphorylation rate constant ( k 2) was 2.66 min −1, and the dissociation constant ( K d) was 4.08 × 10 −8 M. The enhanced inhibitory potency of the α-thioalkylphosphonates may be due to anchimeric assistance of the S atom in the phosphorylation step, or to enhanced stability of the EI complex by hydrogen bonding.