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(•)(+), DPPH(•) and reactive oxygen and nitrogen species: superoxide anion ([Formula: see text]), hypochlorous acid (HOCl) and nitric oxide (NO(•)) 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(•)(+) (50% effective concentration (EC(50)) = 16.1 ± 1.2 μg/mL) and DPPH(•) (EC(50) = 13.8 ± 0.8 μg/mL). The extract demonstrated effective ability to scavenge [Formula: see text] (EC(50) = 52.6 ± 2.1 μg/mL), NO(•) (EC(50) = 48.4 ± 6.8 μg/mL) and HOCl (EC(50) = 714.1 ± 31.4 μg/mL). The extract inhibited peroxyl radical-induced hemolysis (EC(50) = 11.5 ± 1.5 μg/mL), TBARS formation (EC(50) = 38.0 ± 11.7 μg/mL) and hemoglobin oxidation (EC(50) = 186.3 ± 29.7 μg/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.