In vitro glucuronidation of xanthohumol, a flavonoid in hop and beer, by rat and human liver microsomes

Xanthohumol and related prenylflavonoids from hops and beer: to your good health!

Xanthohumol (3'-[3,3-dimethyl allyl]-2',4',4-trihydroxy-6'-methoxychalcone) is the principal prenylated flavonoid of the female inflorescences of the hop plant ('hops'), an ingredient of beer. Human exposure to xanthohumol and related prenylflavonoids, such as 8-prenylnaringenin and isoxanthohumol, is primarily through beer consumption. Xanthohumol has been characterized a 'broad-spectrum' cancer chemopreventive agent in in vitro studies, while 8-prenylnaringenin enjoys fame as the most potent phytoestrogen known to date. These biological activities suggest that prenylflavonoids from hops have potential for application in cancer prevention programs and in prevention or treatment of (post-)menopausal 'hot flashes' and osteoporosis. Xanthohumol and 8-prenylnaringenin are metabolized into many flavonoid derivatives with modified 3,3-dimethyl allyl (prenyl) moieties. Xanthohumol is formed in lupulin glands by a specialized branch of flavonoid biosynthesis that involves prenylation and O-methylation of the polyketide intermediate chalconaringenin. Although a lupulin gland-specific chalcone synthase is known, the aromatic prenyltransferase and O-methyltransferase participating in xanthohumol have not been identified. The prenylflavonoid pathway is a possible target for breeding or biotechnological modification of hops with the aim of increasing xanthohumol levels for beer brewing and 8-prenylnaringenin levels for pharmaceutical production.

Metabolism of 8-prenylnaringenin, a potent phytoestrogen from hops (Humulus lupulus), by human liver microsomes

The female flowers of hops are used throughout the world as a flavoring agent for beer. Recently, there has been increasing interest in the potential estrogenic properties of hop extracts. Among the possible estrogenic compounds in hops, 8-prenylnaringenin is perhaps most significant due to its high in vitro potency exceeding that of other known phytoestrogens. Since data regarding the pharmacokinetic properties of this compound are lacking, we investigated the in vitro metabolism of 8-prenylnaringenin by human liver microsomes. A total of 12 metabolites were identified, and biotransformation occurred on the prenyl group and the flavanone skeleton. The major site of oxidation was on the terminal methyl groups, and of the two possible isomers, the transisomer was more abundant. The double bond on the prenyl group was also oxidized to an epoxide that was opened by intramolecular reaction with the neighboring hydroxyl group. On the flavanone skeleton, the major site of oxidation was at 3'position on the B ring. Other metabolites included oxidation at carbon-3 as well as desaturation of the C ring to produce 8-prenylapigenin. An unusual hydroxy quinone product formed by ipso hydroxylation of the B ring of 8-prenylnaringenin was also detected. This product was probably an intermediate for the B ring cleavage product, 8-prenylchromone.

Inhibition of Peroxynitrite-Mediated LDL Oxidation by Prenylated Flavonoids: The α,β-Unsaturated Keto Functionality of 2‘-Hydroxychalcones as a Novel Antioxidant Pharmacophore

Prenylated 2'-hydroxychalcones and flavanones from the inflorescences of the female hop plant (Humulus lupulus) were shown to inhibit peroxynitrite-mediated oxidation of low-density lipoproteins (LDL) at low micromolar concentrations. LDL oxidation was induced by the peroxynitrite generator, 3-morpholinosydnonimine (SIN-1), and measured by the formation of conjugated dienes and thiobarbituric reactive substances. Human intake of prenylated chalcones and flavanones is mainly through beer, which contains up to 4 mg/L of these polyphenols. The two main oxidation products obtained by SIN-1 and peroxynitrite treatment of xanthohumol (XN), the principal prenylflavonoid of hops, were the aurone, auroxanthohumol (AUXN), and an endoperoxy derivative of XN, named endoperoxyxanthohumol (EPOX). In addition, the reaction produced smaller amounts of the nitro and nitroso derivatives of XN and EPOX. The formation of these nitrated products was enhanced in the presence of sodium bicarbonate (25 mM). SIN-1-induced formation of AUXN is considered to be a superoxide-mediated reaction, while the structure of EPOX points to a two electron oxidation reaction involving a Michael type addition with peroxynitrite as the nucleophile, followed by cyclization yielding a (1,2)-dioxepin-5-one ring structure. The flavanone isomer of XN, isoxanthohumol (IsoXN), unexpectedly showed a slight prooxidant effect instead of an inhibitory effect on LDL oxidation. Except for the formation of minor nitrated products, IsoXN remained largely unmodified upon treatment with SIN-1/peroxynitrite. Taken together, our results suggest that the alpha,beta-unsaturated keto functionality of chalcones is most reactive toward superoxide and peroxynitrite anions.

Microbial transformation of xanthohumol

Microbial transformation of xanthohumol using the culture broth of Pichia membranifaciens afforded three metabolites, (E)-2"-(2"'-hydroxyisopropyl)-dihydrofurano[2",3":4',3']-2', 4-dihydroxy-6'-methoxychalcone, (2S)-2"-(2"'-hydroxyisopropyl)-dihydrofurano[2",3":7,8]-4'-hydroxy-5-methoxyflavanone and (E)-2"-(2"'-hydroxyisopropyl)-dihydrofurano[2",3":2',3']-4'-hydroxy-5-methoxychalcone.

Identification and Biological Activity of Microbial Metabolites of Xanthohumol

Microbial transformation of xanthohumol using the culture broth of Cunninghamella echinulata NRRL 3655 afforded (2S)-8-[4"-hydroxy-3"-methyl-(2"-Z)-butenyl]-4',7-dihydroxy-5-methoxyflavanone (5) and (2S)-8-[5"-hydroxy-3"-methyl-(2"-E)-butenyl]-4',7-dihydroxy-5-methoxyflavanone (6). Xanthohumol (1) and flavanone 6 as well as (E)-2"-(2"'-hydroxyisopropyl)-dihydrofurano[2",3":4',3']-2',4-dihydroxy-6'-methoxychalcone (2), (2S)-2"-(2"'-hydroxyisopropyl)-dihydrofurano[2",3":7,8]-4'-hydroxy-5-methoxyflavanone (3) obtained with Pichia membranifaciens showed antimalarial activity against Plasmodium falciparum.

Childhood acute lymphoblastic leukemia associated with parental alcohol consumption and polymorphisms of carcinogen-metabolizing genes

BACKGROUND

Limited information is available on the association of parental consumption of alcohol prior to and during pregnancy with the risk of childhood leukemia, as well as for the potentially modifying role of genetic polymorphisms.

METHODS

We conducted a population-based, case-control study of 491 incident cases of acute lymphoblastic leukemia age 0-9 years and matched on age and sex to 491 healthy controls. Cases were identified at tertiary care centers in the Province of Québec between 1980 and 1993. Each parent was interviewed separately about alcohol consumption habits. We also used a case-only design with 186 cases to estimate interaction odds ratios between prenatal exposure and child DNA variants in the GSTM1 and CYP2E1 genes.

RESULTS

The adjusted odds ratio for any maternal consumption during pregnancy was 0.7 (95% confidence interval = 0.5-0.9). The interaction odds ratios for the GSTM1 null genotype during third pregnancy trimester was 2.4 (95% confidence interval = 1.1-5.4); the interaction odds ratio for CYP2E1 variant G-1295C (or allele *5) during the nursing period was 4.9 (95% confidence interval = 1.5-16.7).

CONCLUSIONS

The observed association with maternal alcohol consumption during pregnancy could be due to the potential chemopreventive effects of flavonoids found in wine and beer. These possible effects of alcohol may be at least partially genetically determined, although data are preliminary.

Dietary flavonols: chemistry, food content, and metabolism

The flavonols belong to a large group of compounds called flavonoids, which are diverse in their chemical structure and characteristics. Fruits, vegetables, and beverages such as tea and red wine are major sources of flavonols in the human diet. The daily consumption of flavonols is difficult to estimate because values depend on accurate assessment of feeding habits and flavonol content in foods. Food sources, dietary intakes, and bioavailability of flavonols are strongly influenced by variations in plant type and growth, season, light, degree of ripeness, food preparation, and processing, all of which are discussed. In the past few years, a number of studies on the absorption and metabolism of flavonols in humans have been published and the findings from these studies are reviewed. We do not discuss the health effects of flavonols.