Abstract
Olive leaf is reputed to have many beneficial health-related properties, often attributed to the iridoid glycoside oleuropein. In this work, the composition and antioxidant activity of oleuropein-rich olive leaf extracts prepared by ultrasonication were investigated. In addition, the influence of the extracts on Hep G2 cells subjected to glucose-induced oxidative stress was examined. The prepared extracts acted as antioxidants rich in oleuropein and other polyphenols. Among them, the extract prepared at 20°C using 100% ethanol was the most effective radical scavenger, antioxidant and metal chelator. It also contained the largest amount of oleuropein and other phenolic constituents. The extract prepared at 20°C using 70% ethanol, on the other hand, showed the best yield among the investigated extracts. Furthermore, it was able to preserve the cell-membrane integrity of Hep G2 cells subjected to glucose-induced oxidative stress, as well as to maintain intracellular concentration of glutathione and the activity of glutathione S-transferase on the levels comparable to those observed in non-stressed cells. The presented results demonstrate that oleuropein-rich olive leaf extracts can effectively ameliorate the consequences of glucose-induced oxidative stress in liver cells.