Meroterpenoids with BACE1-inhibitory activity from the fruiting bodies of Suillus bovinus and Boletinus cavipes

Alzheimer disease (AD) is the most common type of dementia and accounts for the largest proportion of dementia cases. The amyloid cascade hypothesis is known for the pathogenesis of AD, in which excessive accumulation of amyloid-β (Aβ) leads to the formation of senile plaques and ultimately to AD. Inhibition of β-secretase (BACE1) may contribute to the treatment of AD by suppressing Aβ production. In this study, we isolated and characterized the activity of new and known BACE1-inhibiting compounds from two mushrooms of the Boletales order, Suillus bovinus and Boletinus cavipes, using a BACE1-inhibitory activity-guided separation approach. Three compounds (1–3) were isolated from Suillus bovinus CHCl₃ extract and three compounds (4–6) were isolated from Boletinus cavipes CHCl₃ extract. Compound 1 was a new compound. The structures were elucidated using MS, IR, and NMR. Compounds 1–6 showed BACE1-inhibitory activity (IC₅₀; 21.2, 17.8, 1.0, 1.6, 23.7, and 22.8 μM, respectively). To examine the structure–activity relationship, we also evaluated the activity of geranylgerniol, farnesol, 2,5-dihydroxy-1,4-benzoquinone and mesaconic acid. These compounds showed no activity, and these results indicate that chain terpenes alone do not show BACE1-inhibitory activity, but only when mesaconic acid or a quinone with a hydroxyl group is bound. In addition, the mode of inhibition of 2 and 3 were competitive and 4 was uncompetitive inhibition, respectively, as determined by analysis of Lineweaver–Burk and Dixon plots.