Pharmacological Mapping of the Acetylcholinesterase Catalytic Gorge in Mosquitoes with Bis(n)-Tacrines

Abstract

New insecticides are needed for control of mosquitoes, such as Anopheles gambiae, the major vector of malaria. Acetylcholinesterase is a proven insecticide target site, but conventional organophosphate and carbamate compounds are plagued by concerns about human toxicity and resistance. A pharmacological approach with novel, bivalent bis(n)-tacrines was used to map the catalytic gorge of this enzyme from human and several mosquito species (Anopheles gambiae, Culex restuans, Aedes aegypti, and Aedes albopictus). We screened bivalent bis(n)-tacrines having methylene linkers from 2-12 carbons in length, where proper spacing would allow for high potency binding via interaction with both the catalytic and peripheral sites on the enzyme. The tacrine monomer had fairly similar potency across species (somewhat less for Culex restuans), indicating a common mode of binding at the catalytic site. A greater maximal potency for a bis(n)-tacrine was observed against human AChE than any of the mosquitoes tested. With the exception of Anopheles gambiae, the mosquitoes showed a clear tether length dependence, with tether length most critical in Aedes aegypti. This finding has implications for identifying the targeted amino acid residues in or near the gorge. Despite the greater potency of bis(n)-tacrines against vertebrate than mosquito acetylcholinesterase, the information gleaned from this study should help inform the molecular design of selective anticholinesterase insecticides in other chemical series.