Glucurono- and Sulfo-Conjugation of Kaempferol in Rat Liver Subcellular Preparations and Cultured Hepatocytes

Abstract

The health benefits of flavonoids as preventive nutrition from coronary heart disease and cancer have received considerable attention since they occur naturally in dietary plants. 1,2) For a better understanding of the pharmacological activity of flavonoids, a thorough knowledge of their metabolism in the body is required. Flavonoids are polyphenols and, therefore, considered to be metabolized mainly as conjugated derivatives by coupling with a glucuronic acid or a sulfuric acid during the phase II biotransformation pathway in vertebrates. There have been several reports [3][4][5][6] on the conjugative metabolism of flavonoids, particularly quercetin which is one of the most widely distributed flavonoids found in fruits and vegetables. 7,8) The conjugation of quercetin is somewhat complicated since it also undergoes methylation of its catechol moiety on the B-ring in addition to glucurono-and sulfo-conjugation in the body. Quercetin metabolites are found as glucurono-or sulfo-conjugates of quercetin and isorhamnetin, a 3Ј-O-methylated form of quercetin, in the plasma of rats and humans administered quercetin.3-6) However, details of the different glucurono-and sulfo-conjugated forms of quercetin and isorhamnetin are still poorly understood. In the present studies, the metabolism of kaempferol lacking one of the ortho-dihydroxyl substitutions on the Bring of quercetin was investigated using rat liver subcellular preparations and cultured hepatocytes. Kaempferol was only glucurono-and sulfo-conjugated but not o-methyl-conjugated in the cultured hepatocytes. Thus, kaempferol appeared to be a more suitable model than quercetin for studying glucurono-and sulfo-conjugated forms of flavonols. Glucurono-and Sulfo-Conjugation by Liver Preparations Liver microsomes and cytosol were prepared in 0.25 M sucrose from male Wistar rats weighing 180-220 g provided with standard rat chow and water ad libitum as described by De Duve et al. MATERIALS AND METHODS Materials9) The reaction mixtures for glucurono-conjugation consisted of 100 mM flavonoid, 50 mM Tris-HCl buffer pH 7.4, 5 mM MgCl 2 , 0.02% Brij, 3 mM UDPGA, and 100 mg microsomes in a final volume of 0.5 ml. The reaction mixtures for sulfo-conjugation consisted of 100 mM flavonoid, 50 mM Tris-HCl buffer pH 7.4, 5 mM MgCl 2 , 10 mM dithiothreitol, 600 mM PAPS, and 1 mg cytosol in a final volume of 0.5 ml. Incubations were at 37°C with shaking for 30-120 minutes. Protein was determined by the method of Lowry et al. 10)Glucurono-and Sulfo-Conjugation by Cultured Hepatocytes Hepatocytes were isolated by the collagenase perfusion method 11) from male Wistar rats maintained as described above. These cells were resuspended in Eagle's MEM containing dexamethasone (1 mM), insulin (0.1 mM), triiodothyronine (1 mg/ml), d-aminolevulinic acid hydrochloride (0.2 mM) and 10% fetal calf serum. The cells were seeded in 35 mm plastic culture dishes at a density of 1ϫ10 6 cells/dish, and placed in an incubator in an atmosphere of 5% CO 2 95% air at 37°C. The monolayers of hepatocytes were *...

Pharmacokinetics and Metabolism of the Flavonoid Scutellarin in Humans After a Single Oral Administration

“…The effect of conjugation on biological activities remains to be investigated. No sulfoconjugates were detected in this experiment by LC/MS n and specific enzymatic hydrolysis (␤-glucuronidase-sulfatase/D-saccharic acid 1,4-lactone) methods (Yodogawa et al, 2003), probably because of the inhibitory effect of scutellarein on phenol sulfotransferase (Walle et al, 1995). There has been some controversy regarding how flavonoids are absorbed, and it has been suggested that some flavonoids, mainly those with glucose molecules, might be absorbed intact (Hollman et al, 1999).…”

“…Although the in vitro and in vivo experiments revealed that the glucurono-or sulfo-conjugated metabolites could occur at any hydroxyl group of flavonoids, the 7-OH position on the A-ring is predominantly conjugated to form a glucuronide or sulfate compared with 3-and 5-OH positions (Boersma et al, 2002;O'Leary et al, 2003;Yodogawa et al, 2003;Gradolatto et al, 2004; (Zhang and Brodbelt, 2004), the major metabolites in rat urine were 4Ј-Oglucuronide, followed by 7-O-glucuronide. Because of the low acidity, the 5-OH position was well known to be the least reactive for glucuronidation.…”

Inhibition of P-Glycoprotein Enhances the Suppressive Effect of Kaempferol on Transformation of the Aryl Hydrocarbon Receptor

“…It is known that flavonols undergo metabolism, i.e., glucuronidation and/or sulfation in the body. 19) A conjugation position-specific effect of quercetin metabolites on AhR transformation has been reported. 20) Thus not only kaempferol aglycone but its metabolites might suppress AhR transformation in vivo.…”

Simultaneous determination by UPLC-MS/MS of seven bioactive compounds in rat plasma after oral administration of Ginkgo biloba tablets: application to a pharmacokinetic study

“…However, other studies have shown that flavonoids exist as aglycones, glucuronic acid conjugates, and sulfonic acid conjugates in plasma (Manach et al, 1996;Yodogawa et al, 2003;Silberberg et al, 2005). So the plasma samples should be acid hydrolytic to transform conjugates to aglycones.…”