Antioxidant Activity of Isoflavones and Their Major Metabolites Using Different in Vitro Assays

Corinna E. Rüfer† and Sabine E. Kulling*‡
Institute of Nutritional Physiology, Federal Research Centre for Nutrition and Food, Haid-und-Neu-Strasse 9, 76131 Karlsruhe, and Institute of Nutritional Science, Department of Food Chemistry, University of Potsdam, Arthur-Scheunert-Allee 114-116, 14558 Nuthetal, Germany
J. Agric. Food Chem., 2006, 54 (8), pp 2926–2931
DOI: 10.1021/jf053112o
Publication Date (Web): March 18, 2006
Copyright © 2006 American Chemical Society
†

 Federal Research Centre for Nutrition and Food.

,
*

 To whom correspondence should be addressed. Tel:  +49-33200/ 88-580. Fax:  +49-33200/88-582. E-mail:  kulling@rz.uni-potsdam.de.

,
‡

 University of Potsdam.

Abstract

Isoflavone phytoestrogens found mainly in soybeans and clover are widely studied phytochemicals. Genistein and daidzein, the major isoflavones found in soy, have received the most attention. However, they undergo extensive metabolism in the intestine and the liver, which might affect their biological properties, e.g. their antioxidant capacities. Furthermore, the biological activities of other naturally occurring isoflavones, for instance, glycitein from soy or biochanin A from red clover, have not yet been studied in detail. The aim of this study was to investigate the antioxidant activities of six naturally occurring isoflavones and their corresponding oxidative and bacterial metabolites. The oxygen radical absorbance capacity assay as well as the in vitro oxidation of low density lipoproteins with the conjugated diene and the thiobarbituric acid reacting substances formation as end points were used. The oxidative metabolites of genistein and daidzein as well as equol exhibited the highest antioxidant activities in all three assays. With few exceptions, they were more effective than the positive controls quercetin and ascorbic acid. Formononetin, the 4‘-O-methyl ether of daidzein, showed the lowest antioxidant property. Because the antioxidant efficacy of isoflavones as effective antioxidants is evident at concentrations well within the range found in the plasma of subjects consuming soy products, this biological activity could be of physiological relevance.

Keywords: Isoflavones; bacterial and oxidative metabolites; antioxidant activity; ORAC assay; in vitro oxidation of low-density lipoproteins

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History

  • Published In Issue April 19, 2006
  • Received for review December 13, 2005. Accepted February 16, 2006. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

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