Prenylated Flavonoids from Epimedium koreanum Nakai and their Human Neutrophil Elastase Inhibitory Effects

Human neutrophil elastase (HNE) has been linked to immune system functions, including inflammation, as well as the development of skin wrinkles, suggesting that HNE inhibitors have many practical applications. In this study, the methanol extract of Epimedium koreanum Nakai contained potent HNE inhibitory flavonoids (1–4) with IC 50 values of 6.06–125.80 μM. Prenyl groups within these flavonoids were critical for the inhibitory effects. For example, epimedokoreanin B (1) (IC 50 = 6.06 μM) showed 6-fold higher efficacy than that of its parent luteolin (IC 50 = 36.01 μM). A similar trend was observed for compound 2 (IC 50 = 6.28 μM) and its parent, apigenin (IC 50 = 37.94 μM). Lineweaver–Burk plots showed that the compounds exhibit mixed-type inhibition. In a detailed kinetic study, compound 1 was assigned to mixed type I and exhibited the greatest binding to the free enzyme (K I = 11.7 μM, K IS = 40.9 μM).