Withanolides, a new class of natural cholinesterase inhibitors with calcium antagonistic properties

The withanolides 1-3 and 4-5 isolated from Ajuga bracteosa and Withania somnifera, respectively, inhibited acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BChE, EC 3.1.1.8) enzymes in a concentration-dependent fashion with IC50 values ranging between 20.5 and 49.2 μM and 29.0 and 85.2 μM for AChE and BChE, respectively. Lineweaver-Burk as well as Dixon plots and their secondary replots indicated that compounds 1, 3, and 5 are the linear mixed-type inhibitors of AChE, while 2 and 4 are non-competitive inhibitors of AChE with Ki values ranging between 20.0 and 45.0 μM. All compounds were found to be non-competitive inhibitors of BChE with Ki values ranging between 27.7 and 90.6 μM. Molecular docking study revealed that all the ligands are completely buried inside the aromatic gorge of AChE, while compounds 1, 3, and 5 extend up to the catalytic triad. A comparison of the docking results showed that all ligands generally adopt the same binding mode and lie parallel to the surface of the gorge. The superposition of the docked structures demonstrated that the non-flexible skeleton of the ligands always penetrates the aromatic gorge through the six-membered ring A, allowing their simultaneous interaction with more than one subsite of the active center. The affinity of ligands with AChE was found to be the cumulative effects of number of hydrophobic contacts and hydrogen bonding. Furthermore, all compounds also displayed dose-dependent (0.005-1.0 mg/mL) spasmolytic and Ca2+ antagonistic potentials in isolated rabbit jejunum preparations, compound 4 being the most active with an ED50 value of 0.09 ± 0.001 mg/mL and 0.22 ± 0.01 μg/mL on spontaneous and K+-induced contractions, respectively. The cholinesterase inhibitory potential along with calcium antagonistic ability and safe profile in human neutrophil viability assay could make compounds 1-5 possible drug candidates for further study to treat Alzheimer's disease and associated problems.