Ursolic acid (3β-hydroxy-urs-12-en-28-oic acid) isolated from many medicinal plants has diverse pharmacologically important properties, including strong anti-inflammatory activity. However its interaction with proinflammatory PLA2 is not known. Ursolic acid inhibited secretory PLA2 (sPLA2) enzymes purified from Vipera russelli, Naja naja venom and human pleural fluid and synovial fluid. IC50 values determined for these enzymes ranged from 12 to 18 μM. Group II secretory PLA2 from both venoms & human inflammatory source were found to be sensitive to inhibition in comparison with group I cobra venom sPLA2. Variation in Ca2+ concentration from 2.5 -15 mM did not alter the level of inhibition. Similarly sPLA2 inhibition by ursolic acid is independent of substrate concentration. Ursolic acid interacts with purified venom sPLA2 enzymes and enhances relative fluorescence intensity in a dose dependent manner. In the presence of ursolic acid apparent shift in the far UV-CD spectra of sPLA2 was observed, indicating a direct interaction with the enzyme and formation of enzyme-ursolic acid complex. This complex results in irreversible inhibition of sPLA2 as evident by dialysis study. Inhibition of sPLA2 induced mouse paw edema and indirect hemolytic activity confirmed its sPLA2 inhibitory activity in vivo and in situ respectively. These studies revealed that the strong anti-inflammatory activity of ursolic acid is by inhibiting sPLA2 enzymes.
Keywords: hemolytic activity, Edema, fluorescence spectroscopy, enzyme-ursolic acid complex, Dialysis