Soluble Epoxide Hydrolase Inhibitory Constituents from the Heartwood of Toxicodendron vernicifluum: Isolation, Kinetic Characterization, Molecular Modeling, and Quantitative Analysis

Soluble epoxide hydrolase (sEH) is a therapeutic target for managing inflammation by preserving anti-inflammatory epoxy fatty acids (EpFAs). This study investigates the sEH inhibitory potential of secondary metabolites isolated from the heartwood of Toxicodendron vernicifluum (Stokes) F.A. Barkley. Bioactivity-guided fractionation of the ethanol extract revealed that the ethyl acetate-soluble fraction possessed significant sEH inhibitory activity (~ 60% at 50 µg/mL). Subsequent purification yielded 11 polyphenolic compounds, which were identified via spectroscopic methods and quantified using a validated Ultra-High-Performance Liquid Chromatography (UPLC) protocol. Among these, the aurone sulfuretin ( 7 ) and the flavonol fisetin ( 5 ) exhibited potent competitive inhibition with IC ₅₀ values of 8.8 ± 0.3 µM and 9.6 ± 0.8 µM, respectively. The chalcone butein ( 9 ) demonstrated strong non-competitive inhibition (IC ₅₀  = 21.4 ± 1.5 µM). Molecular docking and 100 ns molecular dynamics (MD) simulations revealed that sulfuretin forms a stable high-affinity complex within the sEH catalytic pocket, anchored by persistent hydrogen bonds with Asp335 and Tyr383. Conversely, butein interacts with a peripheral allosteric site. These findings highlight T. vernicifluum heartwood as a source of diverse sEH inhibitors with potential for development as anti-inflammatory agents.