Some inhibition kinetic properties and in vivo inhibition of the plasma juvenile hormone esterase from the cabbage looper (Trichoplusia ni Hübner) by one phosphoramidothioate and two trifluoromethylketones were examined. O-ethyl,S-phenyl phosphoramidothioate was shown to react irreversibly with the enzyme in a time-dependent manner, and the inhibition reaction can be factored into a reversible step with a dissociation constant, Kd, of 4.55 × 10-5 M followed by a phosphorylation step with a rate constant, k2, of 1.98 min-1. The phosphorylated enzyme did not show spontaneous recovery after 48 hr of dialysis. On the other hand, the two trifluoromethylketones were shown to act as reversible inhibitors, as their inhibited enzyme was regenerated completely after dialysis. However, 1,1,1,-trifluoro-3-thiooctylpropan-2-one, in contrast to 1,1,1-trifluorotetradecan-2-one, showed progressive time-dependent inhibition, and its reaction with the enzyme followed characteristic bimolecular second-order kinetics with a rate constant, ki, of 3.37 × 107 M-1 min-1. The in vivo inhibition data of topically treated larvae at equimolar amounts of the tested compounds indicated rapid penetration, and the stability of the inhibition was higher for the phosphoramidothioate than for the trifluoromethylketones. The relationship of the mechanism of inhibition and the in vivo inhibition of these compounds to the understanding of the interactions between juvenile hormone and juvenile hormone esterase is discussed.
ASJC Scopus subject areas
- Agronomy and Crop Science