Isolation and identification of an antioxidant flavonoid compound from citrus-processing by-product

Background: Large amounts of citrus by-products are released from juice-processing Plants every year. Most bioactive compounds are found in the peel and inner white pulp. Flavonoids are a widely distributed group of bioactive compounds. The methanolic extract of citrus peel powder has been shown to possess strong antioxidant activity. Therefore the aim of this study was to isolate the major antioxidant flavonoid compound from Citrus unshiu (satsuma) peel as citrus by-product and evaluate its antioxidant activity.

Results: The major flavonoid isolated from C. unshiu peel was identified as quercetagetin. The structure of the compound was determined by tandem mass spectrometry and ultraviolet spectroscopy. Its antioxidant activity was assessed by assays of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical, hydroxyl radical and intracellular Reactive Oxygen Species (ROS) scavenging and DNA damage inhibition. Quercetagetin showed strong DPPH radical-scavenging activity (IC₅₀ 7.89 µmol L⁻¹) but much lower hydroxyl radical-scavenging activity (IC₅₀ 203.82 µmol L⁻¹). Furthermore, it significantly reduced ROS in Vero cells and showed a strong protective effect against hydrogen peroxide-induced DNA damage.

Conclusion: The results of this study suggest that quercetagetin could be used in the functional food, cosmetic and pharmaceutical industries.