Mode of inhibition of acetylcholinesterase by monoterpenoids and implications for pest control

Recent studies proved that monoterpenoids could be an alternative to synthetic insecticides against stored-product pests. For that reason, it is necessary to learn the mode of action of these monoterpenoids. Inhibition of acetylcholinesterase (AChE) activity as a possible mode of action by 8 monoterpenoids which cause high mortality of three stored-product insect pests, Sitophilus oryzae L. (Coleoptera: Curculionidae), Rhyzopertha dominica Fabricius (Coleoptera: Bostrichidae) and Cryptolestes pusillus Schönherr (Coleoptera: Cucujidae) and the role of these monoterpenoids as inhibitors were examined. Inhibition of AChE was measured by colorimetric method where a chemical reaction with enzyme acetylcholinesterase, a substrate (acetylthiocoline iodide), the Ellman's reagent and each monoterpenoid as inhibitor was carried out. The majority of monoterpenoids tested inhibited the enzyme acetylcholinesterase; fenchone, S-carvone and linalool produced the highest inhibition. Furthermore, it was observed that fenchone, γ-terpinene, geraniol and linalool showed a reversible competitive inhibition at least at the enzyme's hydrophobic active site. S-carvone, estragole and camphor produced a mixed inhibition for this enzyme binding to either the free enzyme or the enzyme–substrate complex which links to a different site from the active site where the substrate binds. No inhibition of enzyme acetylcholinesterase by E-anethole was observed.