Effectiveness of Prenyl Group on Flavonoids from Epimedium koreanum Nakai on Bacterial Neuraminidase Inhibition

In this study, the inhibitory potential of bacterial neuraminidase (NA) was observed on the leaves of Epimedium koreanum Nakai, which is a popular ingredient in traditional herbal medicine. This study attempted to isolate the relevant, responsible metabolites and elucidate their inhibition mechanism. The methanol extraction process yielded eight flavonoids (1–8), of which compounds 7 and 8 were new compounds named koreanoside F and koreanoside G, respectively. All the compounds (1–8) showed a significant inhibition to bacterial NA with IC(50) values of 0.17–106.3 µM. In particular, the prenyl group on the flavonoids played a critical role in bacterial NA inhibition. Epimedokoreanin B (compound 1, IC(50) = 0.17 µM) with two prenyl groups on C8 and C5′ of luteolin was 500 times more effective than luteolin (IC(50) = 85.6 µM). A similar trend was observed on compound 2 (IC(50) = 0.68 µM) versus dihydrokaempferol (IC(50) = 500.4 µM) and compound 3 (IC(50) = 12.6 µM) versus apigenin (IC(50) = 107.5 µM). Kinetic parameters (K(m), V(max), and K(ik)/K(iv)) evaluated that all the compounds apart from compound 5 showed noncompetitive inhibition. Compound 5 was proven to be a mixed type inhibitor. In an enzyme binding affinity experiment using fluorescence, affinity constants (K(SV)) were tightly related to inhibitory activities.