Abstract
Quercetin, the most abundant flavonoid in the diet, reduces blood pressure and restores
endothelial dysfunction in hypertensive animals. However, quercetin (as the aglycone)
is usually not present in plasma, but it is rapidly metabolised during absorption
by methylation, glucuronidation and sulfation. Depending on the experimental conditions,
quercetin can show anti-oxidant or pro-oxidant effects. We have analysed the pro-oxidant
effects of quercetin and its methylated (3-methylquercetin or isorhamnetin), sulfated
(quercetin 3′-sulfate), glucuronidated (quercetin 3-glucuronide) and methylated plus
glucuronidated (isorhamnetin 3-glucuronide) metabolites. Auto-oxidation, O2
- release and NO scavenging were analysed by means of absorption spectra, lucigenin
chemiluminescence or superoxide dismutase inhibitable cytochrome C reduction and an
amperometric electrode, respectively. The biological activity of NO was tested in
rat aortic rings. Quercetin, isorhamnetin and quercetin 3′-sulfate auto-oxidized in
aqueous buffer and generated superoxide radical. Quercetin but not the glucuronide
scavenged NO. In contrast, the glucuronides were without effect. Quercetin, but not
quercetin 3-glucuronide, inhibited the biological activity of NO. These data indicate
that, in contrast to quercetin, its main circulating forms, i. e., the glucuronides,
do not exert pro-oxidant effects.
Abbreviations
DMSO: dimethyl sulfoxide
NO: nitric oxide
O2
- : superoxide
ROS: reactive oxygen species
Key words
Quercetin - conjugated metabolites - nitric oxide - pro-oxidant and antioxidant effects
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Prof. Francisco Pérez Vizcaino
Department of Pharmacology
School of Medicine
Universidad Complutense
28040 Madrid
Spain
Phone: +34-91-394-1477
Fax: +34-91-394-1465
Email: fperez@med.ucm.es