Selective cholinesterase inhibitory activities of a new monoterpene diglycoside and other constituents from Nelumbo nucifera stamens

A new β -cyclogeraniol diglycoside (5), along with four known components, cycloartenol (1), p-hydroxybenzoic acid (2), vanilloloside (3), and 5′-O-methyladenosine (4), were first isolated from the n-BuOH fraction of Nelumbo nucifera stamens. The chemical structure of 5 was elucidated as 1-hydroxymethyl-2,6,6-trimethyl-1-cyclohexene 9-O-β-D-glucopyranosyl- (1→6)-β -D-glucopyranoside (nuciferoside) on the basis of chemical and spectroscopic evidence, including 1D, 2D NMR, and MS. The anti-Alzheimer effects of 1-5 were evaluated via the acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) inhibition assays. Compounds 1-3 and 5 showed good and noncompetitive inhibition against AChE with IC₅₀ values of 11.89, 20.07, 4.55, and 3.20μM and Ki values of 15.71, 25.44, 7.76, and 5.76μM, respectively. Compounds 1, 2, and 5 also possessed BChE inhibitory activities with IC50 values of 13.93, 62.29, 205.78, and 83.06μM, respectively. The selectivity index (SI) values of 1, 2, 3, and 5, calculated from IC₅₀ values of BChE and AChE, were 1.2, 3.1, 45.7, and 26.0. However, all isolated compounds lacked BACE1 inhibition up to 100μM. Therefore, N. nucifera stamens-derived compounds could potentially exert their primary anti-Alzheimer effects as AChE inhibitors rather than BACE1 inhibitors.