Taspine: Bioactivity-Guided Isolation and Molecular Ligand−Target Insight of a Potent Acetylcholinesterase Inhibitor fromMagnoliaxsoulangiana

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Abstract
A bioactivity-guided approach was taken to identify the acetylcholinesterase (AChE, EC 3.1.1.7) inhibitory agent in a Magnolia x soulangiana extract using a microplate enzyme assay with Ellman's reagent. This permitted the isolation of the alkaloids taspine (1) and (−)-asimilobine (2), which were detected for the first time in this species. Compound 1 showed a significantly higher effect on AChE than the positive control galanthamine and selectively inhibited the enzyme in a long-lasting and concentration-dependent fashion with an IC50 value of 0.33 ± 0.07 μM. Extensive molecular docking studies were performed with human and Torpedo californica-AChE employing Gold software to rationalize the binding interaction. The results suggested ligand 1 to bind in an alternative binding orientation when compared to galanthamine. While this is located in close vicinity to the catalytic amino acid triad, the 1−AChE complex was found to be stabilized by (i) sandwich-like π-stacking interactions between the planar aromatic ligand (1) and the Trp84 and Phe330 of the enzyme, (ii) an esteratic site anchoring with the amino side chain, and (iii) a hydrogen-bonding network.