Glucuronidation versus Oxidation of the Flavonoid Galangin by Human Liver Microsomes and Hepatocytes

Abstract

ABSTRACT:In a previous study, we used human liver microsomes for the first time to study cytochrome P450 (P450)-mediated oxidation of the flavonoid galangin. The combination of CYP1A2 and CYP2C9 produced a V max /K m value of 13.6 ؎ 1.1 l/min/mg of protein. In the present extended study, we determined glucuronidation rates for galangin with the same microsomes. Two major and one minor glucuronide were identified by liquid chromatography/mass spectrometry. The V max /K m values for the two major glucuronides conjugated in the 7-and 3-positions were 155 ؎ 30 and 427 ؎ 26 l/min/mg of protein, thus, exceeding that of oxidation by 11 and 31 times, respectively. This highly efficient glucuronidation appeared to be catalyzed mainly by the UDP-glucuronosyltransferase (UGT)1A9 isoform but also by UGT1A1 and UGT2B15. Sulfation of galangin by the human liver cytosol, mediated mainly but not exclusively by sulfotransferase (SULT) 1A1, also appeared to be efficient. These conclusions were strongly supported by experiments using the S9 fraction of the human liver, in which all three metabolic pathways could be directly compared. When galangin metabolism was examined in fresh plated hepatocytes from six donors, glucuronidation clearly predominated followed by sulfation. Oxidation occurred only to a minor extent in two of the donors. This study for the first time establishes that glucuronidation and sulfation of galangin, and maybe other flavonoids, are more efficient than P450-mediated oxidation, clearly being the metabolic pathways of choice in intact cells and therefore likely also in vivo.

Berry Fruits as a Source of Biologically Active Compounds: The Case of Lonicera Caerulea

“…38 ), the absorptive monocarboxylate transporter (MCT), MRP2, but probably also other MRP isoforms, for glucuronide and sulphate conjugates 38,58 . The metabolism of fl avonoids was initially described to be mediated by cytochrome P450 (CYP) enzymes 59 in liver microsomes from induced rats and from humans, but it has never been shown to be important in vivo or in intact cells, where conjugative metabolism may be expected to compete with oxidation 60 . Glucuronic acid conjugates of fl avonoids have been well-documented with respect to both the molecular site of glucuronidation and the UD P-glucuronyltransferase (UGT) isoforms involved 61 .…”

Cancer chemopreventive properties of orally bioavailable flavonoids—Methylated versus unmethylated flavones

“…This clearly can be explained by very poor oral bioavailability of the polyphenols, as has been shown directly in humans for chrysin [8], quercetin [9,10] the tea flavonoids [11,12] and resveratrol [13,14]. Mechanistically, this is related to the free hydroxyl groups of the polyphenols, giving rise to rapid intestinal/hepatic conjugation by glucuronidation and/or sulfation and excretion [15].…”

Methylated Flavonoids Have Greatly Improved Intestinal Absorption and Metabolic Stability

“…This is clearly due to very low oral bioavailability, as has been shown in clinical studies of some of the more prominent polyphenols, for example, chrysin (5,7-dihydroxyflavone) (Walle et al, 2001), resveratrol (3,5,4Ј-trihydroxystilbene) (Goldberg et al, 2003;Walle et al, 2004), and quercetin (3,5,7,3Ј,4Ј-pentahydroxyflavone) (McAnlis et al, 1999;Williamson and Manach, 2005). Mechanistically, this can be related to the free hydroxyl groups of most polyphenols, giving rise to very rapid conjugation by glucuronidation and sulfation (Otake et al, 2002).…”