Oranges are rich sources of flavonoids that are bioactive and may protect against age-related diseases. The absorption of orange flavanones may be affected by factors such as processing and subject anthropometric variables, and the bioactivity of the absorbed phytochemicals depends on how they are metabolised during absorption. In a randomised cross-over study, twenty subjects consumed a single portion of orange fruit (150 g) or juice (300 g) that contained the flavanones narirutin and hesperidin, and an additional 109 subjects across a broad age range (18–80 years) consumed the juice. Flavanone metabolites were measured in regularly collected samples of plasma and urine. After consumption of fruit or juice, flavanone conjugates, but not the aglycones, were detected in plasma and urine. The flavanone conjugates were shown to include the 7- and 4′-O-monoglucuronides of naringenin, the 7- and 3′-O-monoglucuronides of hesperetin, two hesperetin diglucuronides and a hesperetin sulfo-glucuronide, but no aglycones or rutinosides. Analysis of the plasma pharmacokinetic and urinary excretion data on a dose-adjusted basis indicated no difference in absorption or excretion of either flavanone between the fruit and juice matrices. In the extended urinary excretion dataset the individual variation was very large (range 0–59 % urinary yield). There was a small but significant (P<0·05) decrease in the excretion of hesperetin (but not naringenin) with increasing age (P<0·05), but the effects of sex, BMI and contraceptive pill use were shown not to be associated with the variation in flavanone excretion.

Metal complexation and tandem mass spectrometry were used to differentiate C-and O-bonded flavonoid monoglucoside isomers. Electrospray ionization of solutions containing a flavonoid glycoside and a metal salt led to the generation of the key [M(II) (L) (L-H)] ϩ complexes, where M is the metal ion and L is the flavonoid glycoside. Thirteen flavonoid monoglucosides were examined in combination with Ca(II), Mg(II), Co(II), Ni(II), and Cu(II). Collisional activated dissociation (CAD) of the [M(II) (L) (L-H)] ϩ complexes resulted in diagnostic mass spectra, in contrast to the CAD mass spectra of the protonated, deprotonated, and sodium-cationized flavonoid glucosides. Five common sites of glycosylation could be predicted based on the fragmentation patterns of the flavonoid glucoside/magnesium complexes, while flavonoid glucoside/calcium complexes also were effective for location of the glycosylation site when MS 3 was employed. Cobalt, nickel and copper complexation had only limited success in this application. The metal complexation methods were also applied for characterization of a flavonoid rhamnoside, and the dissociation pathways of the metal complexes indicate that flavonoid rhamnosides have distinctive dissociation features from flavonoid glucosides. (J Am Soc Mass Spectrom 2004, 15, 1287-1299

A three-part tandem mass spectrometric strategy that entails MS n analysis and a post-column LC-MS cobalt complexation method is developed to identify flavonoid monoglucuronide metabolites synthesized using the 1A1 isozyme of human UDP-glucuronosyltransferase (UGT). Ten flavonoid aglycons were used as substrates, spanning the subclasses of flavones, flavonols, and flavanones. The products were characterized by LC-MS and LC-MS n , with post-column cobalt complexation employed to pinpoint the specific sites of conjugation. The dissociation of complexes of the form [Co(II) (flavonoid glucuronide Ϫ H) (4,7-diphenyl-1,10-phenanthroline) 2 ] ϩ allowed identification of the products and differentiation of isomers. The correlation between glycosylation site and elution order is used to provide additional structural confirmation. Flavonoids lacking a 3= hydroxyl group were glucuronidated only at position 7, while those containing this functionality also formed 3=-O-glucuronides and sometimes 4=-O-glucuronides, thus supporting the conclusion that the presence or absence of the 3=-OH group is the major determinant of the regioselectivity of glucuronidation. Moreover, the specific distribution of multiple glucuronide products ( The currently accepted paradigm involves the consumption of flavonoid glycosides in plant-based food products, deglucosylation in the small intestine by ␤-glucosidase or lactose phloridzin hydrolase, and rapid metabolism by Phase I and (especially) Phase II enzymes [1][2][3]. Glucuronidation and sulfation are important metabolic routes for most flavonoids, while methylation or hydroxylation may also occur depending on the structure of the molecule in question [2,3]. There has also been a report of glutathionerelated metabolites in human urine [4]. As a result of these rapid conjugation reactions, neither the original flavonoid glycosides (except anthocyanins) nor the aglycon forms (except catechins) are found in plasma [5][6][7][8]. Early reports of unmodified flavonoid glycosides circulating in the bloodstream [9 -11] were likely mistaken identifications of flavonoid glucuronides, which have similar chromatographic and ultraviolet (UV) spectroscopic characteristics [7,12]. Flavonoids that fail to be absorbed in the small intestine may be broken down by microflora in the large intestine [1][2][3]. This process may release the free aglycons, which can then be absorbed and metabolized, but mostly results in the release of small phenolic acids, which are expelled in the urine [1][2][3]. Quantitative in vivo studies generally show that only a small percentage of consumed flavonoid glycosides is recovered in urine as conjugated Phase II metabolites [13][14][15]. Walle et al. used 14 Clabeled quercetin to show that up to 81% of the administered dose ultimately is exhaled in the form of carbon dioxide [16]. There remains considerable interest in the conjugated metabolites as they may retain some of the bioactivity of the original molecules [5,17].In spite of breakthroughs in the field of flavonoid metabol...

Noncovalent complexes were used for structural determination and isomer differentiation of flavonoid glucuronides. Several flavonoid glucuronides including naringenin-7-O-glucuronide, synthesized here for the first time, were used as test compounds. Electrospray ionization quadrupole ion trap mass spectrometry with collision-induced dissociation (CID) was used to analyze complexes of the form [Co(II) (L-H) (Aux)]+ and [Co(II) (L-H) (Aux)2]+, in which L is the flavonoid glucuronide and Aux is a phenanthroline-based ligand. These complexes yielded characteristic fragmentation patterns that facilitated assignment of the substitution position of the glucuronides. The methods were adapted to liquid chromatography/tandem mass spectrometry (LC-MS/MS) with postcolumn cobalt complexation and were tested on extracts from biological fluids. The metabolites naringenin-7-O-glucuronide and naringenin-4'-O-glucuronide were detected in human urine following the consumption of grapefruit juice. Isomeric quercetin glucuronides were identified and differentiated after spiking rat plasma at the 1 microM level, proving that the new methods are effective at biologically relevant concentrations.

Flavonoids are biologically active compounds in food with potential health effects. We have used the Caco-2 cell monolayer model to study the absorption and metabolism of two flavonols, a class of flavonoids, specifically kaempferol and galangin. Metabolism experiments allowed identification of 5 kaempferol conjugates: 3-, 7- and 4′-glucuronide, a sulphate and a glucurono-sulphate; and 4 galangin conjugates: 3-, 5- and 7-glucuronides, and a sulphate, using specific enzyme hydrolysis, HPLC-MS, and HPLC with post column metal complexation/tandem MS. Transport studies showed that the flavonols were conjugated inside the cells then transported across the monolayer or effluxed back to the apical side. Sulphated conjugates were preferentially effluxed back to the apical side, whereas glucuronides were mostly transported to the basolateral side. For kaempferol, a small amount of the unconjugated aglycone permeated in both directions, indicating some passive diffusion. When kaempferol-3-glucuronide and quercetin7-sulphate were applied to either side of the cells, no permeation in either direction was observed, indicating that conjugates cannot re-cross the cell monolayer. Formation of apical kaempferol-7- and 4′-glucuronides was readily saturated, whereas formation of other conjugates at the apical side and all at the basolateral side increased with increasing concentration of kaempferol, implying different transporters are responsible at the apical and basolateral sides. The results highlight the important but complex metabolic changes occurring in flavonoids during absorption.