Anti-inflammatory activity of oleanolic acid by inhibition of secretory phospholipase A₂

Oleanolic acid, a triterpenoid known for its anti-inflammatory properties, is commonly present in several medicinal plants. The present study evaluated the effect of oleanolic acid on sPLA₂, a key enzyme in inflammatory reactions. Oleanolic acid inhibited sPLA₂ activities of human synovial fluid (HSF), human pleural fluid (HPF) and Vipera russelli (VRV-PL-V) and Naja naja (NN-PL-I) snake venoms in a concentration-dependent manner. The IC₅₀ values of sPLA₂ from these sources ranged from 3.08 to 7.78 μM. Increasing calcium (Ca²⁺) concentrations from 2.5 to 15 mM and substrate concentration up to 180 nM did not affect the level of inhibition. Oleanolic acid enhanced the relative intrinsic fluorescence intensity of sPLA₂ (VRV-PL-V). In the presence of oleanolic acid, an apparent shift in the far UV-CD spectrum of sPLA₂ was observed. These studies indicate direct interaction with the enzyme and formation of an sPLA₂-oleanolic acid complex. The complex formed resulted in irreversible inhibition of sPLA₂. Oleanolic acid inhibited indirect hemolytic activity and mouse pawedema induced by sPLA₂. Inhibition of in vitro and in vivo sPLA₂ activity by oleanolic acid explains the observed anti-inflammatory properties of several oleanolic acid-containing medicinal plants.