Antioxidant capacity of albumin-bound quercetin metabolites after onion consumption in humans

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  • Murota Kaeko
    Department of Food Science, Graduate School of Nutrition and Bioscience, The University of Tokushima
  • Hotta Azusa
    Department of Food Science, Graduate School of Nutrition and Bioscience, The University of Tokushima
  • Ido Hikaru
    Department of Food Science, Graduate School of Nutrition and Bioscience, The University of Tokushima
  • Kawai Yoshichika
    Department of Food Science, Graduate School of Nutrition and Bioscience, The University of Tokushima
  • Moon Jae-Hak
    Department of Food Science, Graduate School of Nutrition and Bioscience, The University of Tokushima College of Agricultural and Life Sciences, Chonnam National University
  • Sekido Keiko
    Department of Fundamental Nursing, School of Health Sciences, The University of Tokushima
  • Hayashi Hiroki
    Kagome Research Institute, Kagome Co. Inc.
  • Inakuma Takahiro
    Kagome Research Institute, Kagome Co. Inc.
  • Terao Junji
    Department of Food Science, Graduate School of Nutrition and Bioscience, The University of Tokushima

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Abstract

Quercetin is a major dietary flavonoid found in onions and other vegetables. It is known that dietary quercetin is metabolized in the intestinal mucosa and the liver and is present as its glucuronide/sulfate conjugates with or without methylation. Although quercetin is known to possess strong antioxidant activity, there are only limited reports on the antioxidant activity of its metabolites. In this study, the antioxidant capacity of quercetin metabolites under physiological conditions was investigated. After consumption of cooked onion, more than 80% of quercetin metabolites were localized in the human plasma fraction containing concentrated serum albumin. Other lipoprotein fractions contained only small amounts of quercetin metabolites. Addition of quercetin 3-O-β-glucuronide to the lipoprotein-eliminated plasma fraction generated antioxidant activity against LDL oxidation in a dose-dependent manner. However, onion consumption failed to enhance the antioxidant activity of the lipoprotein-eliminated plasma fraction against LDL oxidation, probably because the amount of quercetin metabolites bound to albumin was less than the effective level in an ex vivo study. The physiological role of plasma albumin in retaining quercetin metabolites needs to be further clarified. J. Med. Invest. 54: 370-374, August, 2007

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