Evaluation of antioxidant activities of principal carotenoids available in water spinach (Ipomoea aquatica)

A fundamental study of free radical-scavenging activity of three carotenoids purified from water spinach (Ipomoea aquatica cv. Slim leaf) by thin layer chromatography and identified by high performance liquid chromatography–mass spectroscopy (HPLC–MS), namely violaxanthin, lutein and β-carotene, was carried out by measuring the ability to scavenge 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals and 2,2-azobis-3-ethylbenzthiazoline-6-sulphonic acid (ABTS) radicals and by measuring their ability to inhibit red blood cell homelysis and lipid peroxidation. From the results obtained, it can be seen that the three carotenoids, in the range of 0–8 μg/mL, possessed significant antioxidant activity which increased in a concentration-dependent manner. Among them, violaxanthin, a 5,6-epoxycarotenoid, presented the excellent scavenging capacity, and the IC50 values were 8.77, 3.38, 5.06 and 4.25 μg/mL for each assay, respectively. β-Carotene showed highest scavenging percentage in DPPH free radical assay, whereas violaxanthin was most efficient in the other three assays, the results observed suggested that the relationship between structure and antioxidant activities in vitro was complex, epoxidation of the 5,6-ring may improve the antioxidative activities of the carotenoid in the four detected assays in vitro except the quenching of DPPH radicals.