Anti-Acetylcholinesterase Compounds Isolated from the Leaves of Kigelia africana (LAM) Benth (Bignoniaceae)

Acetylcholinesterase (AChE) is an enzyme that is involved in the breakdown of some neurotransmitters. Its inhibition is one of the treatment strategies employed in the management Alzheimer diseases. Flavonoids isolated from the leaves of Kigelia africana were investigated for their comparative AChE inhibition.

The extract of the leaves was subjected to vacuum liquid chromatography (VLC) to obtain four fractions using n-hexane (n-hex, 100%), n-hexane/dichloromethane (hex/DCM, 1:1), dichloromethane/ethyl acetate (DCM/EtOAc, 1:1) and ethyl acetate/methanol (EtOAc/MeOH, 1:1). The four fractions were subjected to AChE inhibitory study with DCM/EtOAc (1:1) fraction showing the highest inhibitory activity. Three flavonoids were isolated from this fraction and their structures were elucidated and characterised using 1D- and 2D-nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) techniques. Their spectroscopic data compared well with literature.

The compounds demonstrated considerable inhibition of AChE activity with luteolin (1), rutin (2) and quercetin (3) that showed IC50 of 945.0, 282.1, 254.8 μg/ml respectively as against the IC50 of 38.93 μg/ml for rivastigmine, a well-known cholinesterase inhibitor. Compound 3 showed 17.89 ± 0.57 and 7.70 ± 0.64 μ/l/mg protein at 200 and 400 μg/ml respectively, for AChE activity as against 10.37 ± 0.99 and 6.24 ± 1.24 μ/l/mg protein showed by rivastigmine at 200 and 400 μg/ml respectively.

This study showed that the constituents responsible for the AChE inhibition in the crude extract as reported by Falode et al., 2017 resided in the DCM/EtOAc (1:1) fraction. The structure-activity relationship of the flavonoids revolves around substitution in position 3 of the compounds.