Hysteresis of insect acetylcholinesterase

Pre-steady-state catalytic properties of insect acetylcholinesterase (AChE, EC 3.1.1.7) were studied with the neutral substrate N-methylindoxylacetate. Kinetics of soluble Apis mellifera and Drosophila melanogaster AChE forms showed lags ( v i = 0 ) before reaching the steady-state. Results were interpreted in terms of slow equilibrium between two conformational states E and E′ of insect AChE. Hysteresis of insect AChE has been pointed out for the first time. The hysteretic behaviour was found to depend on the NMIA concentration and the nature of the enzyme. The maximum induction times ( τ max) to reach the steady-state were 800 and 1000 s with soluble AChE from A. mellifera and D. melanogaster, respectively. The orders of magnitude of the τ max were high and similar to human AChE and BuChE.