Anti-inflammatory and Antiphytopathogenic Fungal Activity of 2,3-seco-Tirucallane Triterpenoids Meliadubins A and B from Melia dubia Cav. Barks with ChemGPS-NP and In Silico Prediction

Two new rearranged 2,3-seco-tirucallane triterpenoids, meliadubins A (1) and B (2), along with four known compounds, 3–6, were isolated from the barks of Melia dubia Cav. Compound 2 exhibited a significant inflammatory inhibition effect toward superoxide anion generation in human neutrophils (EC50 at 5.54 ± 0.36 μM). It bound to active sites of a human inducible nitric oxide synthase (3E7G) through interactions with the residues of GLU377 and PRO350, which may benefit in reducing the neutrophilic inflammation effect. The ChemGPS-NP interpretation combined with bioactivity assay and in silico prediction results suggested 2 to be an agent for targeting iNOS with different mechanisms as compared to a selected set of current approved drugs. Moreover, compounds 1 and 2 showed remarkable inhibition against the rice pathogenic fungus Magnaporthe oryzae in a dose-dependent manner with IC50 values of 137.20 ± 9.55 and 182.50 ± 18.27 μM, respectively. Both 1 and 2 displayed interactions with the residue of TYR223, a key active site of trihydroxynaphthalene reductase (1YBV). The interpretation of 1 and 2 in the ChemGPS-NP physical-chemical property space indicated that both compounds are quite different compared to all members of a selected set of reference compounds. In light of demonstrated biological activity and in silico prediction experiments, both compounds possibly exhibited activity against phytopathogenic fungi via a novel mode of action.