Bioassay-guided isolation of BACE1 inhibitors from Crataegus pinnatifida

Background: Crataegus pinnatifida, functional food has been used in traditional Chinese medicine due to its significant pharmacological effects on various metabolic disorders, neurodegenerative diseases, as well as anti-cancer and anti-viral properties. Despite numerous studies reporting its anti-Alzheimer's disease (AD) activity, there is currently no research on the potential of its extract or constituents to inhibit β-site amyloid precursor protein cleaving enzyme 1 (BACE1). Methods: The study aimed to evaluate the inhibitory effects of the methanolic extract of C. pinnatifida and its solvent-soluble fractions on cholinesterase and β-site amyloid precursor protein cleaving enzyme 1 (BACE1). Following a bioassay-guided isolation, we identified several compounds, including two triterpenoids [corosolic acid (1), euscaphic acid (2)], β-sitosterol glucoside (3), three flavonoids [astragalin (4), hyperoside (5), isovitexin (6)], and four phenolic acid derivatives [gentisic acid (7), protocatechuic acid (8), p-hydroxybenzoic acid (9), p-coumaric acid (10)] from EtOAc fraction. Additionally, two triterpenoids (3-epicorosolic acid (11) and ursolic acid (12)) and β-sitosterol (13) were isolated from the CH2Cl2 fraction, while isoorientin (14) and hymenoside X (15) were isolated from n-BuOH fraction. Notably, compounds 2, 7, 10, and 14 were reported for the first time from the fruit of C. pinnatifida. Results: We observed that 3-epicorosolic acid (11) exhibited promising inhibitory activity against BACE1 with IC₅₀ values of 40.12 ± 1.72 µM. Hyperoside (5) showed moderate activity with IC₅₀ values of 98.50 ± 4.20 µM, while corosolic acid (1) and astragalin (4) displayed mild BACE1 inhibitory activity, with IC₅₀ values of 179.55 ± 1.57 and 192.30 ± 5.14 µM, respectively. We also investigated the structure activity relationship of ursane-type triterpenoids based on in vitro and in silico results, which revealed that the hydroxyl groups position of the triterpenes have a significant impact on their potency. Our findings suggest that α‑hydroxyl group at the C-2 and C-3 positions and the absence of α‑hydroxyl group at the C-19 position are essential for BACE1 inhibitory activity. Conclusions: This study highlights the significance of the configuration and position of hydroxyl groups and their impact on BACE1 inhibitory activity. The findings of this study could be leveraged to develop novel therapeutic approaches for Alzheimer's disease.