Synergistic effect on tyrosinase inhibition of Cassia fistula flower with Gynura procumbens leaf extracts

The antioxidants and tyrosinase inhibitors found in plants are promising agents in skincare products. Cassia fistula and Gynura procumbens are tropical plants found in many Southeast Asian countries. This study evaluated the antioxidant and tyrosinase inhibitory activities of ethanol extracts prepared from Cassia fistula flowers and Gynura procumbens leaves. The antioxidant activity of Cassia fistula flower and Gynura procumbens leaf extracts were investigated via the stable free radical 2,2-diphenyl-1-picrylhydrazyl method. The tyrosinase inhibitor effects of extracts and its combination against tyrosinase were examined with L-DOPA as substrate. The synergistic effect of Cassia fistula flower and Gynura procumbens leaf extracts combination was demonstrated by using the combination index method based on the median-effect principle that allows quantitative determination of its combination, where CI < 1, =1, and >1 indicates synergism, additive effect and antagonism, respectively. The CompuSyn software was used for determining synergism and antagonism at all concentrations or effects levels simulated automatically. The antioxidant activity of Cassia fistula flower extract was 2.7-fold higher than Gynura procumbens leaf extract, but the tyrosinase inhibitor activity was lower. The combination of Cassia fistula extract and Gynura procumbens extract in ratio 7:1.2 demonstrated synergistic effects when the inhibition was greater than 50 % with the CI ranged 0.90 to 0.49 for the fractional inhibition of Fa = 0.50–0.95. In conclusion, this combination demonstrated a synergistic effect and may be used further in cosmetic formulation.