NEW METABOLIC PATHWAYS FOR FLAVANONES CATALYZED BY RAT LIVER MICROSOMES Dejan Nikolic and Richard B. van Breemen Department of Medicinal Chemistry and Pharmacognosy, UIC/National Institutes of Health Center for Botanical Dietary Supplements Research, College of Pharmacy, University of Illinois at Chicago, Chicago, Illinois (Received August 20, 2003; accepted December 18, 2003) This article is available online at http://dmd.aspetjournals.org ABSTRACT: Flavonoids represent a diverse group of natural pigments widely distributed in the plant kingdom and are an important component of human diet due to their high content in fruits and vegetables. Since many flavonoids have been shown to be potent inhibitors, substrates, and even inducers of various cytochrome P450 iso- forms, there is considerable interest in studying interactions of this class of molecules with the cytochrome P450 enzyme system. In this study, the metabolism of several simple flavanones by rat liver microsomes was investigated and compared. In addition to the expected aromatic hydroxylation products, several novel meta- bolic pathways were observed including C-ring desaturation to form the corresponding flavones, oxidation of the B-ring to gener- ate an unusual quinol oxidation product, B-ring cleavage with the formation of chromone derivatives, and reduction of carbonyl group to form flavan-4-ol derivatives. The metabolites were char- acterized and identified primarily by using liquid chromatography- tandem mass spectrometry with comparison to authentic stan- dards. Formation of flavones from dietary flavanones might have biological significance since flavones often exhibit pharmacologi- cal activities that are different from those of flavanones. However, little is known about the pharmacological activities of the other types of flavonone metabolites. Flavonoids are a diverse group of natural products encompassing more than 5000 compounds. Chemically, flavonoids contain a com- mon phenylchromanone structure (C 6 -C 3 -C 6 ) with at least one hy- droxyl group substituent or a hydroxyl derivative such as a methoxy group. Flavonoids are classified based on the level of oxidation and substitution pattern of the C-ring, whereas members within a class differ in the pattern of substitution on the A- and B-rings. Plants often synthesize O-linked or C-linked glycosides of these compounds. Present in significant quantities in fruits and vegetables, flavonoids are an important component of the human diet. Estimates of daily intake range from 28 mg/day in the Netherlands (Hertog et al., 1993) to 1 g/day in the United States (Kuhnau, 1976). Many beneficial pharmacological properties have been attributed to flavonoids includ- ing antioxidant, anti-inflammatory, anticarcinogenic, chemopreven- tive, and cytochrome P450-inhibitory activities. Although less abundant in plants than some other classes of fla- vonoids, many flavanones also exhibit important biological activities. Major dietary sources of flavanones include citrus fruits and juices, propolis, and honey. Naringenin, one of the most abundant flavanones in citrus fruits, has been shown to possess antioxidant (Wang and Goodman, 1999), antiproliferative (Manthey and Guthrie, 2002), and weakly estrogenic activities in vitro (Miksicek, 1993). Erlund et al. (2002) estimated that a high vegetable diet can provide up to 132 mg of hesperetin and 29 mg of naringenin per day, and that the plasma level of naringenin can reach 113 nM. The prenylated analog of naringenin, 8-prenylnaringenin, is a much more potent phytoestrogen (Milligan et al., 1999). Propolis and honey contain pinocembrin and pinobanksin, as well as several other flavanones. The methylated derivative of pinocembrin is called pinostrombin and has recently been shown to be a potent inducer of quinone reductase (Fahey and Stephenson, 2002). For comprehensive recent reviews of other phar- macological activities of flavonoids see Ross and Kasum (2002) and Havsteen (2002). Recently, there has been considerable interest in studying the in- teractions of flavonoids with cytochrome P450 isozymes, since this process has the potential to interfere with the metabolism of various drugs (see Hodek et al., 2002). Although early studies indicated that many flavonoids could act as cytochrome P450 inhibitors, much of this activity may be attributed to the competitive metabolism of flavonoids. Some flavonoids such as diosmin and flavanone have also been shown to be inducers of cytochrome P450 enzymes (Canivenc- Lavier et al., 1996; Ciolino et al., 1998). Another important aspect of flavonoid metabolism is that their structural similarities can result in metabolic interconversion. Since metabolism might transform one class of flavonoid into another, new pharmacological activity might result. Indeed, a comprehensive study by Nielsen et al. (1998) showed that many methoxylated flavonoids can be demethylated to their oxidized analogs. For example, hesperetin is O-demethylated to form naringenin, or kaempferide is metabolized to kaempferol (Otake and Walle, 2002). Nielsen et al. (1998) showed that ring oxidation is Supported by Grant P50AT00155 provided to the UIC/National Institutes of Health Center for Botanical Dietary Supplements Research by the Office of Dietary Supplements, the National Institute of General Medical Sciences, the Office for Research on Women’s Health, and the National Center for Comple- mentary and Alternative Medicine. The contents are the responsibility of the authors and do not necessarily represent the official views of the sponsors. Address correspondence to: Richard B. van Breemen, Department of Me- dicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, 833 S. Wood St., Chicago, IL 60612-7231. E-mail: breemen@uic.edu 0090-9556/04/3204-387–397$20.00 DRUG METABOLISM AND DISPOSITION Vol. 32, No. 4 Copyright © 2004 by The American Society for Pharmacology and Experimental Therapeutics 1264/1136403 DMD 32:387–397, 2004 Printed in U.S.A. 387 at ASPET Journals on April 9, 2017 dmd.aspetjournals.org Downloaded from