Changes in membrane phospholipids, identified by Arrhenius plots of acetylcholinesterase and associated with pyrethroid resistance (kdr) in houseflies (Musca domestica)

Abstract

AbstractMembrane‐bound acetylcholinesterase (AChE), extracted from the heads of susceptible and knockdown‐resistant (kdr) houseflies, was assayed at a range of temperatures between 5 and 35°C. Arrhenius plots using the results showed discontinuities commonly associated with membrane‐bound enzymes and generally attributed to an abrupt change in the viscosity of the lipids as they undergo a phase change at a temperature characteristic of their chemical composition. The enzyme from susceptible flies had a transition temperature of 14°C compared with 19°C and 21°C for the enzymes from kdr and super‐kdr strains, respectively. This variation was associated with a progressive decrease in activation energies, both above and below the transition temperature, with increasing resistance. The enzyme from a hybrid between susceptible and super‐kdr flies had the same properties as that from susceptible flies, corresponding with the genetically fully‐recessive behaviour of this resistance mechanism in toxicological studies. After digestion with phospholipase A2, the AChE from super‐kdr flies gave unchanged Arrhenius plots, whereas with the susceptible enzyme, both transition temperature and activation energies changed markedly to resemble more closely the enzyme from kdr flies. These results are discussed in relation to the likely changes in phospholipid composition and its effect on the conformation of the, as yet unidentified, target(s) of pyrethroids and DDT.