Purification and antioxidant activities of soybean isoflavones

Abstract

An efficient procedure was developed and used to extract and purify isoflavones from defatted soy flour. The purity of the final product reached 94%. The antioxidant activities of soybean isoflavones (genistein and daidzein purchased from Sigma Chemical Co, and a mixture of isoflavone glycosides extracted from defatted soy flour) were investigated by a cholesterol oxidation model accelerated by 2,2’-azobis(2-methylpropionamidine) dihydrochloride (AAPH). All components exhibited very high antioxidant activities in the inhibition of cholesterol oxidation. Genistein had higher inhibition to the formation of 5,6beta-epoxycholesterol (5,6b-EP) and 7-ketocholesterol (7-keto), while no significant difference was found between the antioxidant activities of daidzein and genistein against cholesterol oxidation to 7beta-hydroperoxycholesterol (7b-OOH) and 5,6alpha-epoxycholesterol (5,6a-EP). The glycosides mixture had characteristics similar to genistein and daidzein in preventing cholesterol oxidation, but the glycosides mixture was found to have lower antioxidant activities compared to aglycones. The biological activities of genistein and daidzein to affect cellular response in SVEC4-10 mouse endothelial cells to t-butyl hydroperoxide-induced oxidative stress were also evaluated. Genistein had higher cytotoxicity than daidzein to SVEC4-10 cells and none exerted discernable antioxidant activities in this cell culture model. The antioxidant activities of soybean isoflavones may contribute to their potential role in reducing the formation of certain types of cancers, so daidzein might be a more appropriate cancer preventive neutraceutical because of its lower cytotoxicity. In view of the high cytotoxicity, this cell culture model might not be suitable for the study of antioxidant activity of soybean isoflavones. Some modifications or another cell culture model is needed in order to further this study of antioxidant activity. Defatted soy flour is a by-product of oil extraction, so the antioxidant activities of soybean isoflavones could improve the utilization of the by-product and benefit the soy industry.