In vitro synergistic antioxidant activity of binary combinations of plant extracts and synthetic compounds

The aim of present study was to find possible synergistic effect on antioxidant activity in vitro in the binary combinations of methanolic leaf extracts of six medicinal plants namely, Callistemon lanceolatus (Sm.) Sweet, Ocimum gratissimum L., Cymbopogon winterianus Jowitt ex Bor., Cymbopogon flexuosus (Nees ex Steud.) W.Watson, Mentha longifolia (L.) L., and Vitex negundo L.and also in their combinations with synthetic antioxidants i.e. butylated hydroxytoluene and butylated hydroxyanisole. 1, 1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging assay was used to determine antioxidant activity of different extracts which showed IC₅₀ values in the range of 39.77–360.32 μg/mL. Interaction studies were performed by calculating the fractional inhibitory concentration index (FIC₅₀I) of each combination and isobolographic analysis. Results revealed an approximately 2 fold reduction in the IC₅₀ values of extracts in their binary combinationsand3-4 fold reduction in the IC₅₀ values of BHT and BHA in their synergistic combinations with extracts. However combinations of BHT and BHA with leaf extracts of V. negundo and C. flexuosus showed additive and antagonistic effects, respectively. Exploitation of synergistic interactions in the combinations of natural and synthetic antioxidants is a promising approach to produce more effective antioxidants from the existing ones. Reduction in the active doses of synthetic antioxidants enables their use at safe concentrations in food preservation.