Human neutrophil elastase inhibitory potential of flavonoids from Campylotropis hirtella and their kinetics

Campylotropis hirtella is used as a food supplement in the subtropical region of China. In an intensive hunt for human neutrophil elastase inhibitors, we isolated eight flavonoids from C. hirtella three of which (1-3) emerged to be elastase inhibitors. Geranylated flavonoids (1-3) displayed significant inhibitory activity with IC₅₀s between 8.5 and 30.8 μM. The most striking example was geranylated isofavanone 3 that inhibited elastase significantly (IC₅₀=30.8 μM) but its parent compound (dalbergioidin) and isoflavone analog (5) were inactive (IC₅₀>200 μM). Compounds (1-3) displayed different kinetic mechanisms (noncompetitive, competitive, and mixed type, respectively) that were dependent upon the parent skeleton. The competitive inhibitor, isoflavan-3-ol-4-one 2 manifested an inhibition of isomerization profile for elastase with kinetic parameters K₅=0.0386 M^-1S^-1, K₆=0.0244 μM^-1S^-1 and K_i^app = 16.3427 μM. The specific identification of metabolites was accomplished by LC-DAD-ESI/MS that was also used to analyze abundance of active components (1-3) within the plant.