In vitro antioxidant activity of olive leaf extract (Olea europaea L.) and its protective effect on oxidative damage in human erythrocytes

Aims: This study aimed to evaluate in vitro antioxidant capacity of olive leaf extract (OLE), Olea europaea L., and its protective effect on peroxyl radical-induced oxidative damage in human erythrocytes. Main methods: The OLE was evaluated by the following assays: i) total phenolic and flavonoid content; ii) oleuropein content; iii) Ferric reducing antioxidant power (FRAP); iv) antioxidant activity against ABTS(center dot+) , DPPH center dot and reactive oxygen and nitrogen species: superoxide anion (O-2(center dot-)), hypochlorous acid (HOCl) and nitric oxide (NO center dot) and v) protective effect on peroxyl radical-induced oxidative damages in human erythrocytes as hemolysis, thiobarbituric acid reactive substances (TBARS) formation and oxyhemoglobin oxidation. Key findings: Total phenolic and flavonoid contents were 131.7 +/- 9.4 mg gallic acid equivalents/g dry weight (dw) and 19.4 +/- 1.3 mg quercetin equivalents/g dw, respectively. Oleuropein content was 25.5 +/- 5.2 mg/g dw. FRAP analysis was 281.8 +/- 22.8 mg trolox equivalent/g dw and OLE inhibited ABTS(center dot+) (50% effective concentration (EC50) = 16.1 +/- 1.2 mu g/mL) and DPPH center dot (EC50 = 13.8 +/- 0.8 mu g/mL). The extract demonstrated effective ability to scavenge O-2(center dot-) (EC5(0) = 52.6 +/- 2.1 mu g/mL), NO center dot (EC50 = 48.4 +/- 6.8 mu g/mL) and HOCl (EC50 = 714.1 +/- 31.4 mu g/mL). The extract inhibited peroxyl radical-induced hemolysis (EC50 = 11.5 +/- 1.5 mu g/mL), TBARS formation (EC50 = 38.0 +/- 11.7 mu g/mL) and hemoglobin oxidation (EC50 = 186.3 +/- 29.7 mu g/mL) in erythrocytes. Significance: OLE is an important source of natural antioxidants; it has effective antioxidant activity against different reactive species and protects human erythrocytes against oxidative damage.