Oleanolic acid and ursolic acid stabilize liposomal membranes.

The effects of oleanolic acid (OA) and ursolic acid (UA) on the fluidity and stability of dipalmitoyl phosphatidylcholine (DPPC) liposomal membrane were monitored by measuring the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene labeled in the liposomal membrane and the leakage of calcein from the probe-encapsulated liposomes. The experiments with the liposomes made of DPPC and OA or UA showed that OA and UA exhibited a moderate fluidity-modulating effect for the liquid-crystalline liposomal membrane, and a strong condensing effect for both crystalline and liquid-crystalline liposomal membranes. Their effects were comparable to those of cholesterol. These results suggest that their fluidity-modulating and condensing effects might have some implications in their biological functions.