Butenolides and triterpenoids from the fungus Pallidohirschioporus biformis (syn. Trichaptum biforme): isolation, structure determination and bioactivity profile

Pallidohirschioporus biformis is a common annual polypore distributed in subtropical to temperate regions of the Northern Hemisphere. Extensive mycochemical analysis of the methanol extract of P. biformis resulted in the isolation of eight secondary metabolites (1-8). The structure determination of the fungal metabolites was carried out by one- and two-dimensional NMR and MS analysis. Among them biformolide (1) is a new natural product, while compound 2 (9-(2-methoxy-3, 4-dimethyl-5-oxofuran-2-yl)nonanoic acid) is reported from natural sources for the first time. Besides compounds 1-2, two other butenolides, namely hydroxydihydrobovolide (3) and schumannione (4), 4-hydroxy-trans-2-nonenoic acid (5), betulin (6), and two ergostane-type derivative, ergosterol peroxide (7) and ergosterol peroxide glucoside (8) have been identified. The isolates as well as the chloroform, n-hexane, and methanol extracts of P. biformis were evaluated for their potential bioactivities in several in vitro assays, including hyaluronidase, acetyl- and butyrylcholinesterase (AChE and BuChE), alpha-glucosidase, antibacterial, tyrosinase inhibitory and antioxidant tests. The bioassay results for the three fungal extracts suggest that hyaluronidase could be a potential enzymatic target of P. biformis extracts. Of the secondary metabolites tested, compound 8 showed the highest hyaluronidase inhibitory activity (91%), whereas compound 6 exhibited moderate inhibition (32%), and the remaining compounds demonstrated little to no activity. The anticholinesterase assays revealed a notable difference between the inhibitory effects of P. biformis on AChE and BuChE. Although the chloroform and methanol extracts exhibited significant BuChE inhibition, they showed no inhibitory activity against AChE. Our findings identify hydroxydihydrobovolide (3) as the primary bioactive compound responsible for the observed inhibition of butyrylcholinesterase activity. This study is the first comprehensive report on the chemical composition of wild-growing P. biformis sporocarps, providing an in-depth investigation into the isolation and structural characterization of the species' most distinctive biologically active compounds.