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

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-beta-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.

Suppressive effect of quercetin on acute stress-induced hypothalamic-pituitary-adrenal axis response in Wistar rats

“…A total of 4 h after feeding, quercetin and its methylated metabolites (isorhamnetin or tamarixetin) were detected at 35-119 ng/g and 7-72 g/g, respectively, in acidhydrolyzed brain homogenate. Furthermore, recent studies have demonstrated that quercetin metabolites possess a variety of biological effects, such as anti-oxidative, anti-inflammatory and anti-atherosclerotic actions [32][33][34][35]. In addition, we have found that quercetin metabolites accumulate in rat brain 30 min after oral administration of quercetin at 50 mg/kg BW (Y. Kawai, A. Ishisaka, S. Saito and J. Terao, unpublished data).…”

Multifaceted preventive effects of single agent quercetin on a human prostate adenocarcinoma cell line (PC-3): implications for nutritional transcriptomics and multi-target therapy

“…More specifically, it has been well established that quercetin possesses potent inhibitory activity against multidrug-resistance phenotype [70], angiogenesis [71], proliferation [72], cell cycle progression [73] and carcinogen activation [74]. Besides, quercetin has been suggested a potent antioxidant [75], a robust apoptosis inducer [24] and a promoter of differentiation in cancer cells [76]. Furthermore, lack of cytotoxicity of quercetin has provided the rationale to examine it in clinical trials against various types of malignancies [77].…”

β-Glucuronidase activity and mitochondrial dysfunction: the sites where flavonoid glucuronides act as anti-inflammatory agents

“…(21–23) The concentrations of the total quercetin metabolites in human plasma after the intake of quercetin (150 mg from onion) are around 1 µM at maximum. (24) We also measured the concentration of Q3GA in human plasma to be 264 nM. (23) In contrast to the observation that no quercetin aglycone was detected in human plasma, tea catechins, especially ECg and EGCg, can be detected as the aglycones (20 and 60 ng/ml, respectively, at maximum concentrations 1–2 h after ingestion) in human plasma after intake of green tea.…”