title: Metabolic fate of [2-14C]quercetin-4'-glucoside in rats and in cultured cells
Creators: name: Graf, Brigitte A.
Publisher: name: University of Glasgow
Year: 2004
Subjects: subject: Cellular biology.

description: Background: Flavonoids are a class of phytochemicals which have been associated with several health benefits, including reduced risk of cancer and cardiovascular disease. Quercetin is one of the most ubiquitous dietary flavonoids with estimated intakes of 3-15 mg/day. The bioactivity of dietary constituents is dependent upon their bioavailability, but the absorption and biotransformation of quercetin is poorly understood as reflected by estimates of absorption rates from &lt;0.1% to 50%. Further investigations of the putative health effects of quercetin require more reliable data on its bioavailability and metabolism in the gastrointestinal tract (Gl-tract) and its distribution to body tissues. Section I and II; Using a radiolabelled quercetin glycoside, the oral absorption and subsequent body distribution of [2-14C]quercetin-4'-glucoside and its metabolites were traced in Rowett Hooded Lister rats. Based on the radioactivity content of plasma and body tissues, -10% of the oral dose (7.6 mg/kg BW) was absorbed and appeared in tissues almost exclusively in the form of &gt;20 different methylated, glucuronated, and/or sulfated quercetin metabolites. The Gl-tract held &gt;85% of the ingested radioactivity at 0.5, 1, 2 and 5 hours after oral administration; at 2 hours all radiolabelled compounds in the GI tract were metabolised forms of the ingested compound. It was hypothesized that quercetin-4'-glucoside was completely metabolised in the Gl-tract before absorption into the circulation. Biological effects of dietary quercetin may be due to its metabolites. Section III; To investigate the individual contribution of the liver (the primary metabolising organ for xenobiotic compounds), Gl-tissues or Gl-microflora in quercetin metabolism, cultured rat liver (CC-1) and small intestinal (IEC-6) cells were used as a model for pre-systemic metabolism. Both cell lines produced similar types and numbers of quercetin metabolites and 11 of the 16 cell culture metabolites were identical with quercetin metabolites formed in the rat. The degree of chemical instability of [2-14C] quercetin-4'-glucoside in cell culture medium was not anticipated a priori. Less than 0.2% of the administered [2-14C]quercetin-4'-glucoside, its radiolabelled break down products or metabolites were associated with the cells. Despite apparent metabolism in the cell culture medium, the presence of cells was essential for the formation of quercetin metabolites, indicating that metabolites had been formed either on the cell membrane or inside the cells with quick subsequent export from cells into the medium. In conclusion, cell culture experiments may be a useful tool to investigate potential health effects of dietary quercetin and in vivo metabolites, provided 1) chemical break down of quercetin can be controlled and 2) the cell line can synthesize similar metabolites as found in vivo. Section IV; Previous reports indicate quercetin up-regulates glutathione in vitro and in vivo, a mechanism by which this flavonoid may increase cellular antioxidant activity. However, glutathione is also up-regulated by oxidative stress and some studies suggest quercetin may possess pro-oxidant activity. Therefore, it was investigated whether dietary glycosylated quercetin up-regulates glutathione in cells that form in vivo metabolites of quercetin. We found free and glycosylated quercetin had different effects on glutathione regulation, suggesting that the biological activity of these forms of the flavonoid may be different. We now hypothesize that pro-oxidant by-products formed during quercetin oxidation in cell culture medium may be responsible for many of the observed effects of quercetin in vitro.