New coumarins isolated from the stem bark of Fraxinus rhynchophylla inhibit human neutrophil elastase and LPS-induced inflammation in RAW 264.7 cells

Two new coumarins (1 and 2), together with six known compounds (3-8), were isolated from the stem bark of Fraxinus rhynchophylla. The structures of these compounds were established by extensive spectroscopic studies and chemical evidence. The inhibitory activities of the isolated compounds (1-8) against human neutrophil elastase (HNE) were evaluated in vitro; all compounds tested exhibited inhibitory effects against HNE activity, with IC50, values ranging from 3.6-74.3 mu M. Among the compounds tested, compounds 2 and 8 exhibited considerable HNE inhibitory activity, with IC50, values of 3.6 and 9.6 mu M, respectively. The mechanism by which the two active compounds (2 and 8) inhibited HNE was noncompetitive inhibition, with inhibition constant (K-i) values of 2.2 and 6.8 mu M, respectively. Furthermore, compounds 2 and 8 dose-dependently inhibited LPS-induced NO production in RAW 264.7 cells. Consistent with these findings, they also suppressed LPS-induced expression of iNOS and COX-2 at the protein level in RAW 264.7 cells.