Acetylcholinesterase activity of electric eel is increased or decreased by selected monoterpenoids and phenylpropanoids in a concentration-dependent manner

The profitable insecticidal action of monoterpenoids prompted us to test their efficiency against stored-grain beetle species, via inhibition of acetylcholinesterase (AChE). For this, we first studied the ability of the monoterpenoids geraniol, linalool, camphor, fenchone, carvone and gamma-terpinene, besides the phenylpropanoids trans-anethole and estragole to inhibit Electrophorus AChE. The results indicated that while AChE activity increased (15-35%) with 40 mu M geraniol, camphor, gamma-terpinene and linalool, the activity decreased (60-40%) with 5 mM carvone, gamma-terpinene, and fenchone. The Km for AChE was 0.52 +/- 0.02 mM in control assays, which fell to 0.28 +/- 0.01 mM or 0.32 +/- 0.01 mM in assays with 20 mu M linalool or gamma-terpinene added. In the millimolar range, the terpenoids behaved as weak inhibitors. Unexpectedly, AChE inhibition by camphor, carvone, gamma-terpinene, and fenchone gave Hill numbers ranging 2.04-1.57, supporting the idea that AChE was able to lodge more than one monoterpenoid molecule. The plots of 1/v vs. 1/S at varying monoterpenoid provided straight lines; fenchone and gamma-terpinene acting as competitive inhibitors and carvone and camphor as non-competitive inhibitors. Moreover, the secondary plots of the slope K-M(app)/V-max(app) vs. [I] and of 1/V-max(app) vs. [I] gave parabolic curves, which lent support to the proposed capacity of AChE to bind more than one monoterpenoid molecule. The fitting of the curves to a second-order polynomial equation allowed us to calculate the inhibition constants for the interaction of AChE with fenchone, gamma-terpinene, carvone and camphor. The previously unnoticed increase in AChE activity with monoterpenoids should be considered as a reminder when advising the use of essential oils of plants or their constituents as anti-AChE agents to attenuate pathological signs of Alzheimer's disease. (C) 2015 Elsevier Ireland Ltd. All rights reserved.