Which acetylcholinesterase functions as the main catalytic enzyme in the Class Insecta?

Most insects possess two different acetylcholinesterases (AChEs) (i.e., AChEl and AChE2; encoded by ace I and ace2 genes, respectively). Between the two AChEs, AChEl has been proposed as a major catalytic enzyme based on its higher expression level and frequently observed point mutations associated with insecticide resistance. To investigate the evolutionary distribution of AChEl and AChE2, we determined which AChE had a central catalytic function in several insect species across 18 orders. The main catalytic activity in heads was determined by native polyacrylamide gel electrophoresis in conjunction with Western blotting using AChEl- and AChE2-specific antibodies. Of the 100 insect species examined, 67 species showed higher AChEl activity; thus, AChEl was considered as the main catalytic enzyme. In the remaining 33 species, ranging from Palaeoptera to Hymenoptera, however, AChE2 was predominantly expressed as the main catalytic enzyme. These findings challenge the common notion that AChE1 is the only main catalytic enzyme in insects with the exception of Cyclorrhapha, and further demonstrate that the specialization of AChE2 as the main enzyme or the replacement of AChEl function with AChE2 were rather common events, having multiple independent origins during insect evolution. It was hypothesized that the generation of multiple AChE2 isoforms by alternative splicing allowed the loss of ace1 during the process of functional replacement of AChEl with AChE2 in Cyclorrhapha. However, the presence of AChE2 as the main catalytic enzyme in higher social Hymenoptera provides a case for the functional replacement of AChEl with AChE2 without the loss of ace1. The current study will provide valuable insights into the evolution of AChE: which AChE has been specialized as the main catalytic enzyme and to become the main target for insecticides in different insect species. (C) 2012 Elsevier Ltd. All rights reserved.