Molecular and Pharmacokinetic Aspects of the Acetylcholinesterase-Inhibitory Potential of the Oleanane-Type Triterpenes and Their Glycosides

Abstract

The acetylcholinesterase-inhibitory potential of the oleanane-type triterpenes and their glycosides from thebark of Terminalia arjuna (Combreatceae), i.e.,arjunic acid, arjunolic acid, arjungenin, arjunglucoside I, sericic acid and arjunetin, is presented. The studies are based on in silico pharmacokinetic and biomimetic studies, acetylcholinesterase (AChE)-inhibitory activity tests and molecular-docking research. Based on the calculated pharmacokinetic parameters, arjunetin and arjunglucoside I are indicated as able to cross the blood-brain barrier. The compounds of interest exhibit a marked acetylcholinesterase inhibitory potential, which was tested in the TLC bioautography test. The longest time to reach brain equilibrium is observed for both the arjunic and arjunolic acids and the shortest one for arjunetin. All of the compounds exhibit a high and relatively similar magnitude of binding energies, varying from ca. -15 to -13 kcal/mol. The superposition of the most favorable positions of all ligands interacting with AChE is analyzed. The correlation between the experimentally determined IC₅₀ values and the steric parameters of the molecules is investigated. The inhibition of the enzyme by the analyzed compounds shows their potential to be used as cognition-enhancing agents. For the most potent compound (arjunglucoside I; ARG), the kinetics of AChE inhibition were tested. The Michaelis-Menten constant (Km) for the hydrolysis of the acetylthiocholine iodide substrate was calculated to be 0.011 mM.

Keywords: AChE inhibition kinetics; acetylcholinesterase inhibitory test; blood–brain barrier permeation; memory impairment; molecular docking.

Figure 1

The 1/k vs. C_M relationships obtained for the tested triterpenes: arjunic acid (1), arjunolic acid (2), arjungenin (3), arjunglucoside I (4), sericic acid (5) and arjunetin (6) usingthe BMC (above) and SDS (below) systems.

Figure 2

LogBB values obtained for the tested triterpenes: arjunicacid (1), arjunolic acid (2), arjungenin (3), arjunglucoside I (4), sericic acid (5) and arjunetin (6) using biomimetic and computational methods.

Figure 3

Result of the TLC-bioautography assay for the inhibition of the acetylcholinesterase enzyme on the silica gel showing different concentrations of the tested triterpenes: arjunic acid (1), arjunolic acid (2), arjungenin (3), arjunglucoside I (4), sericic acid (5) and arjunetin (6).

Figure 4

(A) The superposition of the most favorable poses of all ligands interacting with AChE. (B) The most favorable location of the arjunglucoside I molecule bound to AChE. The ligand molecule is shown as thick sticks, whereas all of the closest amino-acid residues (of a distance no longerthan 0.4 nm) are represented by thin sticks. The description of the interaction types is given in the text. The residue numbering is compatible with the PDB:3EVE record. (C) Linear correlations between the experimentally determined IC₅₀ values (recalculated as ln(IC₅₀)) and the molecular volume (blue points) or molecular area (red points) of the studied compounds, represented by the dashed or solid line, respectively.

Figure 5

Absorbance (A) vs. time (min) for 0.00045 mM of ARG.

Figure 6

Lineweaver–Burk plot for initial velocity (Vo) vs. substrate concentration (mmol/L).