Appraisal of biological activities and identification of phenolic compound of African marigold (Tagetes erecta) flower extract.

The flowers of African marigold (Tagetes erecta L), a medicinal plant widely cultivated in Thailand, were subjected to evaluation for total phenolics, DPPH scavenging and thiobarbituric acid-reactive substance (TBARs) assays as well as tyrosinase inhibitory activity. In preliminary studies, the ethyl acetate (EA) extract obtained by continuous extraction showed the highest activities with highest phenolic content among all extracts. Bioassay-guided fractionation of EA extract led to isolation of a flavonoid identified as quercetagetin. Interestingly, it was found that quercetagetin exhibited potent DPPH scavenging activity with IC50 of 3.70 μg/ml which is about 2-3 times higher activity than standard quercetin (IC50 5.07 μg/ml) and trolox (IC50 9.93 μg/ml). Moreover, it exhibited tyrosinase inhibitory activity on L-tyrosine (IC50 89.31 μg/ml), higher than α- and β-arbutins (IC50 157.77 and 222.35 μg/ml) and slightly higher (IC50 128.41 μg/ml) than ellagic acid (IC50 151.1 μg/ml) when using L-DOPA as substrate. Testing with skin fibroblasts, all the extracts and quercetagetin demonstrated no toxic effect. These finding strongly indicate that African marigold flower is a promising source of natural antioxidative and tyrosinase inhibitory substances with safe to skin.