Antioxidant and cytoprotective activities of native Australian fruit polyphenols

Four native Australian fruits, Illawarra Plum (Podocarpus elatus Endl., Podocarpaceae), Kakadu Plum (Terminalia ferdinandiana Exell, Combretaceae), Muntries (Kunzea pomifera F. Muell., Myrtaceae) and Native Currant (Acrotriche depressa R.Br., Epacridaceae) were examined for antioxidant and cellular protective activities. Each fruit showed significantly greater antioxidant activity than a blueberry (Vaccinum sp., cv. Biloxi) reference with Kakadu Plum exhibiting 13.3-fold and 2.4-fold activity of blueberry in the ferric ion reducing antioxidant power (FRAP) and oxygen radical absorbance capacity (ORAC-H) assays, respectively. A lyophilised polyphenolic-rich extract of Kakadu Plum exhibited the greatest cellular antioxidant activity (CAA assay) of 71.5 μmol QE/g, and was followed by Illawarra Plum, Native Currant and Muntries (46.3, 20.0 and 14.4 μmol QE/g, respectively). Polyphenolic-rich extracts of Kakadu Plum and Muntries (but not Illawarra Plum and Native Currant) extracts efficiently protected RAW 264.7 cells against hydrogen peroxide induced apoptosis in a dose-dependent manner. Kakadu Plum and Native Currant polyphenolic-rich extracts increased the Nrf2/Keap1 ratio, suggesting activation of the antioxidant response element (ARE) through the Nrf2/Keap1 complex. The results suggest Kakadu Plum exhibits the greatest antioxidant potential, exerting antioxidant activity through free radical scavenging and affecting two (Nrf2/Keap1) downstream transcription factors.