Bio-Assay-Guided Isolation of Fractions and Constituents with Antioxidant and Lipid-lowering Activity from Allium cepa

Active fractions and constituents with antioxidant and lipid-lowering activities were investigated using bio-assay-guided isolation and identification. The data showed that the antioxidant fraction of A. cepa was AC50%, the main constituents of which were quercetin and isoquercitrin, by way of both ultra-high performance liquid chromatography–mass spectrometry (UPLC-MS) and bio-assay-guided purification and elucidation. Similarly, the lipid-lowering active fraction of A. cepa was AC30% with the main constituents of 3,4-dihydroxybenzoic acid and quercetin 3,4′-O-diglucoside. Also, bio-assay-guided isolation led to the isolation and identification of five known compounds with a purity of more than 98%, and quercetin was both the best free radical scavenger and lipid-lowering constituent. Moreover, the mechanism of the lipid-lowering effect of AC30% might be its reduction in mRNA expression levels of sterol regulatory element binding protein 2 (SREBP2) and FAS gene in lipid synthesis. Otherwise, reducing the mRNA expression level of lipid synthesis genes, including SREBP1, SREBP2, fatty acid synthetase (FASN), β-Hydroxy β-methylglutaryl-CoA (HMGCR), stearoyl CoA desaturase 1 (SCD1), and increasing the mRNA expression level of lipid decomposition gene, such as carnitine palmitoyl transferease-1 (CPT1), might be involved in the lipid-lowering activity of quercetin. This study suggested that Allium cepa might be used to prevent and treat oxidative stress and dislipidemia-related disorders, including NAFLD.