Cholinesterase and BACE1 inhibitory diterpenoids from Aralia cordata

Fourteen diterpenes were isolated from the n-hexane fraction of the roots of Aralia cordata (syn. = A. continentalis). Through spectroscopy, the chemical structures were determined as: ent-pimara-8(14),15-diene-19-oic acid (1); ent-kaur-16-en-19-oic-acid (2); 18-nor-ent-pimara-8(14),15-diene-4β-ol (3); 18-nor-ent-kaur-16-ene-4β-ol (4); ent-pimara-8(14),15-diene-19-ol (5); 7α-hydroxy-ent-pimara-8(14),15-diene-19-oic acid (6); 7β-hydroxy-ent- pimara-8(14),15-diene-19-oic acid (7); ent-pimar-15-en-8α,19-diol (8); 7-oxo-ent-pimara-8(14),15-diene-19-oic acid (9); 16α-hydroxy-17- isovaleroyloxy-ent-kauran-19-oic acid (10); 17-hydroxy-ent-kaur-15-en-19-oic acid (11); 15α,16α-epoxy-17-hydroxy-ent-kauran-19-oic acid (12); 16α,17-dihydroxy-ent-kauran-19-oic acid (13); and 16α-methoxy-17- hydroxy-ent-kauran-19-oic acid (14). Compounds 4, 5, 8, 12, and 14 were first isolated from this plant. The anti-Alzheimer and antioxidant effects of ent-pimarane-type diterpenes 1, 3, 5, 8, and 9, as well as ent-kaurane-type diterpenes 2, 4, and 10-13, were evaluated via β-site amyloid precursor protein cleaving enzyme 1 (BACE1), acetylcholinesterase (AChE), butyrylcholinesterase (BChE), peroxynitrite (ONOO^-), and nitric oxide (NO•) assays. Of the compounds tested, 8 exerted the most effective BChE inhibition with an IC₅₀ value of 7.58 μM, followed by 3, 13, 11, 2, and 10. Compounds 9-11 exhibited good BACE1 inhibitory activities with IC₅₀ values of 18.58-24.10 μM. However, 11 showed marginal AChE inhibitory effect, and all compounds tested showed no scavenging activities on ONOO^- and NO• at a concentration of 100 μM.