Indole Metabolites as Modulators of Human Cholinesterases: A Kinetic and Molecular Docking Study with Comparison to Alzheimer's Drugs

Abstract

Objective: Alzheimer's disease (AD) is characterised by the gradual decline of cognitive function and impaired cholinergic neurotransmission due to reduced brain acetylcholine levels. AChE and BChE regulate acetylcholine degradation and are important targets in AD. This research investigated the inhibitory effects of indole-3-propionic acid (IPA) and indole-3-butyric acid (IBA) on human cholinesterases, using enzyme kinetics and molecular docking. Methods: We assessed the inhibitory effects of purified human BChE and commercially sourced erythrocyte-derived AChE using a modified Ellman spectrophotometric assay in a 96-well microplate. Kinetic parameters (Km, Vmax, and Ki) were determined by nonlinear regression and the Michaelis-Menten model, with Lineweaver-Burk plots used to analyse inhibition patterns. IC50 values were calculated by nonlinear regression analysis. Molecular docking simulations were performed to predict the binding positions of the indole derivatives and the reference Alzheimer's drugs donepezil and tacrine analogue in the active sites of AChE and BChE. Results: According to docking simulations, both indole derivatives bind to the catalytic gorge of cholinesterases, but with lower predicted affinity than reference inhibitors. Enzyme inhibition assays showed that IBA exhibited weak inhibitory activity against both enzymes, with IC50 values of 15.61 mM for BChE and 54.28 mM for AChE. IPA exhibited stronger inhibition, particularly against BChE, with an IC50 of 2056 & micro;M. Kinetic analysis indicated that IPA displays mixed-type inhibition of BChE (Km = 170.1 & micro;M, Ki = 1774 & micro;M), while its inhibition of AChE is competitive (IC50 = 3773 & micro;M, Km = 268.5 & micro;M, Ki = 6727 & micro;M). Conclusion: These findings suggest that short-chain indole metabolites can interact with cholinesterases, but are not potent inhibitors. Compared to IBA, IPA showed higher activity and distinct inhibition profiles for BChE and AChE. The development of new indole-based cholinesterase inhibitors may prove to be potent modulators of cholinergic enzymes.