Antioxidant activity of phenolic extracts from different cultivars of Italian onion (Allium cepa) and relative human immune cell proliferative induction.

ABSTRACT

CONTEXT The total antioxidant activity (TAC) may vary considerably between onion cultivars. Immunological effects of onion phenolic compounds are still underestimated. OBJECTIVE: The objective of this study is to determine the total phenol content (TPC) and the relative TAC of three Allium cepa L. (Liliaceae) onion cultivars cultivated in Cannara (Italy) Rossa di Toscana, Borettana di Rovato, and Dorata di Parma, and to evaluate the phenol extracts ability to induce human immune cell proliferation. MATERIALS AND METHODS: TPC was determined by the Folin-Ciocalteu method, TAC with FRAP, TEAC/ABTS, and DPPH methods. Peripheral blood mononuclear cells from healthy human donors were incubated for 24 h at 37 °C with 1 ng/mL of phenolic extract in PBS, immunostained, and then analyzed by 4-color flow cytometry for the phenotypic characterization of T helper cells (CD4+ cells), cytotoxic T lymphocytes (CD8+ cells), T regulatory cells (CD25high CD4+ cells), and natural killer cells/monocytes (CD16+ cells). RESULTS: Rossa di Toscana displayed the highest TPC (6.61 ± 0.87 mg GA equivalents/g onion bulb DW) and the highest TAC with the experienced methods: FRAP, 9.19 ± 2.54 µmol Trolox equivalents/g onion bulb DW; TEAC/ABTS, 21.31 ± 0.41 µmol Trolox equivalents/g onion bulb DW; DPPH, 22.90 ± 0.01 µmol Trolox equivalents/g onion bulb DW. Incubation with Rossa di Toscana extract determined an increase in the frequency of the antitumor/anti-infection NK CD16+ immune cells (23.0 ± 0.4%). DISCUSSION AND CONCLUSIONS: Content of health-promoting phenols and the deriving antioxidant and immunostimulating activity vary considerably among the investigated cultivars. Rossa di Toscana can be considered as a potential functional food.