Competitive neutrophil elastase inhibitory isoflavones from the roots of Flemingia philippinensis

Abstract

Flemingia philippinensis has been used throughout history to cure rheumatism associated with neutrophil elastase (NE). In this study, we isolated sixteen NE inhibitory flavonoids (1-16), including the most potent and abundant prenyl isoflavones (1-9), from the F. philippinensis plant. These prenyl isoflavones (2, 3, 5, 7, and 9) competitively inhibited NE, with IC50 values of 1.3-12.0 mu M. In addition, they were reversible, simple, slow-binding inhibitors according to their respective parameters. Representative compound 3 had an IC50 = 1.3 mu M, k(3) = 0.04172 mu M-1 min(-1), k(4) = 0.0064 min(-1), and K-i(app) = 0.1534 mu M. The K-ik/K-iv ratios (18.5 similar to 24.6) for compound 3 were consistent with typical competitive inhibitors. The prenyl functionality of isoflavones significantly affected inhibitory potencies and mechanistic behavior by shifting the competitive mode to a noncompetitive one. The remaining flavonoids (10-16) were confirmed as mixed type I inhibitors that preferred to bind free enzyme rather than the enzyme-substrate complex. Fluorescence quenching analyses indicated that the inhibitory potency (IC50) closely followed the binding affinity (K-SV). (C) 2018 Elsevier Inc. All rights reserved.