Introduction: As single extracts, Coccinia grandis and Blumea balsamifera have been known to have potent antioxidant activities. However, the synergistic antioxidant effect of the combination of these plant extracts was unknown. In this study, the combination of C. grandis and B. balsamifera extracts was investigated for its antioxidant and synergistic properties. Methods: Separately, C. grandis and B. balsamifera leaves were extracted with ethanol. After evaporation, the thick extracts were assayed for their total phenolic content (TPC) and total flavonoid content (TFC). The antioxidant properties of single and combined extracts were measured using the molybdenum(VI) reducing power, ferric reducing antioxidant power (FRAP), 2,2-diphenyl-1-picrylhydrazyl (DPPH), and 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) methods. The possible synergism effect was evaluated using the checkerboard method and the combination index values were also calculated. Results: The TPC and TFC of the B. balsamifera extracts were much greater than that of C. grandis extract. In the molybdenum(VI) reducing power and FRAP assay, the reducing power of the extract combination increased as B. balsamifera extract concentration increased (P < 0.05). In the ABTS+ and DPPH radical scavenging assays, B. balsamifera extract demonstrated a higher antioxidant activity than C. grandis extract (P < 0.05). When combined, increasing the concentration of B. balsamifera caused an increase in the radical scavenging activity (P < 0.05). Synergism was observed in the combination of the extracts with low concentration ratios. Conclusion: In this study, we showed that the combination of C. grandis and B. balsamifera leaf extracts possessed synergistic antioxidant properties.
Keywords: Checkerboard method, Free radicals scavenger, Antagonism, Combination index, Synergism