BACKGROUND: Oxidative stress has been implicated in the progression of various diseases, which may result in the
depletion of endogenous antioxidants. Exogenous supplementation with antioxidants could result in increased
protection against oxidative stress. As concerns have been raised regarding synthetic antioxidant usage, the
identification of alternative treatments is justified. The aim of the present study was to determine the antioxidant
efficacy of Burkea africana and Syzygium cordatum bark extracts in an in vitro oxidative stress model.
METHODS: Cytotoxicity of crude aqueous and methanolic extracts, as well as polyphenolic-rich fractions, was
determined in C2C12 myoblasts, 3T3-L1 pre-adipocytes, normal human dermal fibroblasts and U937 macrophagelike
cells using the neutral red uptake assay. Polyphenolic content was determined using the Folin-Ciocalteau and
aluminium trichloride assays, and antioxidant activity using the Trolox Equivalence Antioxidant Capacity and DPPH
assays. The extracts efficacy against oxidative stress in AAPH-exposed U937 cells was assessed with regards to
reactive oxygen species generation, cytotoxicity, apoptosis, lipid peroxidation and reduced glutathione depletion.
RESULTS: B. africana and S. cordatum showed enrichment of polyphenols from the aqueous extract, to methanolic
extract, to polyphenolic-rich fractions. Antioxidant activity followed the same trend, which correlated well with the
increased concentration of polyphenols, and was between two- to three-fold stronger than the Trolox antioxidant
control. Both plants had superior activity compared to ascorbic acid in the DPPH assay. Polyphenolic-rich fractions
were most toxic to the 3T3-L1 (IC50’s between 13 and 21 μg/ml) and C2C12 (IC50’s approximately 25 μg/ml) cell
lines, but were not cytotoxic in the U937 and normal human dermal fibroblasts cultures. Free radical-induced
generation of reactive oxygen species (up to 80%), cytotoxicity (up to 20%), lipid peroxidation (up to 200%) and
apoptosis (up to 60%) was successfully reduced by crude extracts of B. africana and the polyphenolic-rich fractions
of both plants. The crude extracts of S. cordatum were not as effective in reducing cytotoxic parameters.
CONCLUSION: Although oxidative stress was attenuated in U937 cells, cytotoxicity was observed in the 3T3-L1 and
C2C12 cell lines. Further isolation and purification of polyphenolic-fractions could increase the potential use of
these extracts as supplements by decreasing cytotoxicity and maintaining antioxidant quality.