Antioxidant activity of polyphenols from solid olive residues of c.v. Coratina

The antioxidant profile of extracts from solid olive residue (SOR) of c.v. Coratina, a cultivar widely diffused in the south of Italy, using both cell-free and cell-based experimental models, was investigated. A total hydroalcoholic extract (polyphenols content 19.7%) and a purified extract (Oleaselect™) (polyphenols content 35.1%) were tested for their ability to quench the stable free radical DPPH, the peroxyl radicals (ORAC assay), by monitoring the loss in fluorescence of R-phycoerythrin induced by the peroxyl radical generator AAPH and their ability to inhibit the cumene hydroperoxide-induced lysis of rat red blood cells (RBC).The total hydroalcoholic extract showed IC50 26.96 ± 1.53 μg/ml in the DPPH assay, that 10 μg/ml were equivalent to 2.11 ± 0.12 μg/ml Trolox (ORAC assay) and IC50 1.7 ± 0.20 μg/ml in the RBC hemolysis. The Oleaselect™ extract was 4 to 5 folds more active than the hydroalcoholic extract in all the experimental models, with IC50 values of 7.36 ± 0.38 μg/ml in the DPPH test and of 0.38 ± 0.03 μg/ml in RBC; the antioxidant activity in the ORAC assay was slightly greater than that of Trolox (10 μg/ml equivalent to 11.45 ± 0.40 μg/ml). The scavenging effect of the extract in the ORAC assay was compared to that of verbascoside (the main polyphenol component) and of caffeic acid (the basic constituent of verbascoside): the results indicate that caffeic acid (10 μg/ml equivalent to 35.70 ± 2.95 μg/ml Trolox) is more potent than verbascoside (10 μg/ml equivalent to 15.42 ± 1.21 μg/ml Trolox) in entrapping peroxyl radicals. Finally the antioxidant activity of the Oleaselect™ extract was confirmed in human umbilical endothelial cells (EC) exposed to the site-specific peroxyl radical inducer AAPH, where a massive lipid peroxidation process (marker the fluorescence probe BODIPY) takes place, paralleled by a marked loss of cell viability (calcein assay). The purified extract (1–20 μg/ml) pre-incubated with EC for 1 h dose-dependently inhibited both the lipid-peroxidation damage and cell death. Taking into account the total polyphenol content, these results clearly indicate a greater antioxidant activity for the purified extract, due to a cooperative antioxidant interaction among its polyphenol constituents.