Total phenolics, antioxidant and xanthine oxidase inhibitory activity of three colored onions (Allium cepa L.)

Abstract

Methanolic extracts from bulbs of three onion (Allium cepa L.) varieties: red (Pusa Red), white (Pusa White Round), and yellow (Arka Pitamber) and the CHCl 3, Et 2O, EtOAc, and n-BuOH fractions obtained from those extracts were investigated for their total phenolics and antioxidant activity. The antioxidant activity measurements, expressed as total antioxidant status (TAS), were 5.2–50.18, 2.2–35.6 and 3.5–42.2 mg ml−1 for red, white and yellow onion bulb extracts, respectively, as compared to trolox (IC 50, 40.2 mg ml−1). The total phenolic content of the samples was 62–186, 56–156 and 68–165 mg GAE ml−1 for red, white and yellow onion bulb extracts, respectively, with all solvents. There was a positive correlation between the total phenolic content and antioxidant activity for MeOH extracts and EtOAc fractions (R2=0.96, 0.8 respectively). Of the selected colored onion bulbs the red onion (IC 50=14.2 μg ml−1) revealed more potent xanthine oxidase inhibitory activity to those of yellow (IC 50=15.5 μg ml−1) and white onion (IC 50=17.0 μg ml−1) in vitro and as compared to control allopurinol (IC 50=10.5 μg ml−1). The kinetic study by xanthine oxidase revealed that the white and yellow onion extracts were non-competitive inhibitors, as Km values were constant, while Vmax consequently decreased with increased inhibitor concentration. In the red onion extract there was an increase in Km values while Vmax remained constant at all extract concentrations, thus confirming a competitive type of inhibition by the Lineweaver–Burk equation. These findings suggest that these onion varieties can be considered as potential sources of antioxidants and XO inhibitory agents.