Xanthohumol and Related Prenylflavonoids from Hops and Beer
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PHYTOCHEMISTRY Phytochemistry 65 (2004) 1317–1330 www.elsevier.com/locate/phytochem Molecules of Interest Xanthohumol and related prenylflavonoids from hops and beer: to your good health! Jan F. Stevens a,*, Jonathan E. Page b,* a Department of Chemistry and the Linus Pauling Institute, Oregon State University, 117 Weniger Hall, Corvallis, OR 97331, USA b Plant Biotechnology Institute, National Research Council of Canada, 110 Gymnasium Place, Saskatoon, Sask., Canada S7N 0W9 Received 23 April 2004; accepted 26 April 2004 Available online 28 May 2004 Abstract Xanthohumol (30-[3,3-dimethyl allyl]-20,40,4-trihydroxy-60-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 pre- vention programs and in prevention or treatment of (post-)menopausal ‘hot flashes’ and osteoporosis. Xanthohumol and 8-pren- ylnaringenin 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 prenyl- transferase 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. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Xanthohumol; 8-Prenylnaringenin; Prenylflavonoids; Flavonoids; Hops; Humulus lupulus; Beer; Antioxidant; Cancer chemoprevention; Phytoestrogen; Breast enhancement 1. Introduction recent years as cancer chemopreventive agents, while 8-prenylnaringenin, an isomerization product of des- Xanthohumol is a structurally simple prenylated methylxanthohumol also present in beer, enjoys fame chalcone that occurs only in the hop plant, Humulus as the most potent phytoestrogen isolated to date. lupulus L. (Cannabaceae), where it is the principal These exciting biological activities may lead to prenylflavonoid of the female inflorescences (usually biomedical application of xanthohumol and 8-pren- referred to as ‘hops’ or ‘hop cones’). Hops are used to ylnaringenin in the future. Furthermore, hop pren- add bitterness and flavor to beer, and therefore the ylflavonoids are currently tested in many academic main dietary source of xanthohumol and related and industrial laboratories for other biological activi- prenylflavonoids is beer. Xanthohumol and other ties. At the same time, hop-containing herbal prepa- prenylated chalcones have received much attention in rations are being marketed for breast enlargement in women, without proper testing of efficacy and toxicity. The use of hops in herbal preparations represents a * Corresponding authors. Tel.: +1-541-737-9534; fax: +1-541-737- small market and does not seem to be promoted by 2062 (J.F. Stevens); tel.: +1-306-975-4187; fax: +1-306-975-4839 (J.E. Page). the hop growing industry. However, hop growers are E-mail addresses: [email protected] (J.F. Stevens), very interested in alternative applications for hops and [email protected] (J.E. Page). its constituents in view of the continuing trend of 0031-9422/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.phytochem.2004.04.025 1318 J.F. Stevens, J.E. Page / Phytochemistry 65 (2004) 1317–1330 decreasing market prices of hops and improving ag- is the result of eastward migration to Japan and ricultural production methods. America and westward to Europe from the center of The purpose of this review is to provide an overview the species’ origin in China during the mid to late of the chemistry, biological activities, and biotechno- Tertiary (Small, 1980; Neve, 1991). logical aspects of xanthohumol and other prenylated flavonoids from hops. Biosynthetically related to the prenylated flavonoids are the prenylated acylphlorog- lucinols, i.e., humulones (a-acids) and lupulones (b- 3. Isolation and chemical synthesis acids), which represent the commercial value of hops as bittering substances for the beer industry but potential Xanthohumol was first isolated, partially character- biomedical applications have not yet been identified for ized and named by Power et al. (1913). They also ob- this group of terpenophenolics. Hops are also rich tained a related substance which they called humulol, sources of flavonol glycosides and condensed tannins: which was later identified as the flavanone, iso- these groups of flavonoids will not be treated either as xanthohumol, by Verzele et al. (1957). In the years fol- they are widely distributed in the plant kingdom. lowing 1957, there remained some doubt as to the position of the prenyl substituent relative to the meth- oxy group in xanthohumol. Two independent groups 2. Distribution and chemotaxonomic significance of xan- were finally able to confirm Verzele’s original structure thohumol and related flavonoids as the correct structure of xanthohumol by partial syn- thesis and chemical degradation studies (Vandewalle, The distribution of xanthohumol is limited to and 1961; Orth and Riedl, 1963). At the time of preparation ubiquitous within H. lupulus. Xanthohumol is secreted of this review, no formal synthesis of xanthohumol as part of the hop resin (‘lupulin’) by glandular tri- could be retrieved from the SciFinder database. The lack chomes found on the adaxial surfaces of cone bracts. It of interest in the synthesis of this compound is probably is also found in the trichomes on the underside of young related to the availability of xanthohumol from sources leaves. Xanthohumol is the main prenylflavonoid of such as CO2-extracted hops, a waste product of the hop- hops (0.1–1% on dry weight); in the resin, xanthohumol processing industry. It is expected that new synthetic is accompanied by at least 13 related chalcones (Table methods will appear in the literature when labeled 1), all of which occur at 10–100-fold lower concentra- xanthohumol is needed for in vivo studies. The recent tions relative to xanthohumol. Most of the chalcones interest in 8-prenylnaringenin as a novel phytoestrogen contain a free 20-hydroxy group and can therefore has spurred the development of a synthesis for 8-pren- isomerize to their corresponding flavanones. The struc- ylnaringenin from naringenin and prenyl alcohol tures of the most abundant prenylated hop flavanones, (Gester et al., 2001). i.e., isoxanthohumol, 6-prenylnaringenin, and 8-pren- The external accumulation of xanthohumol and re- ylnaringenin, are also listed in Table 1. lated prenylflavonoids in lupulin glands significantly In a study to determine whether prenylflavonoid facilitates the extraction of these lipophilic compounds variation had diagnostic value in distinguishing be- (Stevens et al., 1997). Whole hop cones can be rinsed tween hop cultivars, we found that the presence of briefly with chloroform or acetone to dissolve the resin xanthogalenol (Table 1), named after its source constituents. The extract thus obtained contains the H. lupulus cv. ‘Galena’, was strictly limited to a few bulk of the bitter acids and the prenylflavonoids but is varieties that were all derived from H. lupulus cv. largely free of cone tissue constituents. Crude xan- ‘Brewer’s Gold’. The latter variety is the result of an thohumol can be isolated from the resin extract by open-pollination event between a wild female plant column chromatography on Sephadex LH-20 using from Manitoba and an unknown European male at methanol as the eluent. The crude product can be pu- Wye College, England, in 1918 (Neve, 1991). This in- rified by semi-preparative HPLC on a reversed-phase teresting piece of information led us to include many C18-column. This method has been improved in recent wild North American hop plants and one sample of years by extracting hops with acetone after pre-extrac- Humulus cordifolius (originally from Japan) in our tion of hops using supercritical CO2 to remove the bulk survey of 22 flavonoids. Multivariate analysis of the of the bitter acids (supercritical CO2-extraction is an flavonoid profiles obtained from 120 cone and leaf industrial process for the production of bitter acids; the samples yielded two well-resolved groups of hops: (1) wasted hop materials were made available to us). Ad- European hops and (2) Central US and Japanese wild dition of a 5% solution of sodium carbonate in water to hops (Stevens et al., 2000). This dichotomy with regard the concentrated acetone extract usually results in pre- to the presence of xanthogalenol and other 40-O- cipitation of chlorophyll and other fatty materials. The methylated chalcones clearly supports the hypothesis aqueous layer, containing the prenylflavonoids in their that the present-day disjunct distribution of H. lupulus phenolate form, is acidified and extracted with ethyl J.F. Stevens, J.E. Page / Phytochemistry 65 (2004) 1317–1330 1319 Table 1 Prenylated chalcones and flavanones from hops Structure Trivial name Reference Chalcones Power et al. (1913) HO OCH OH Xanthohumol 4' 6' 3 4 Verzele et al. (1957) α Huebner and Riedl (1960) 2' β 1 Haensel and Schulz (1988) OH O R3 R2O OR1 OH R4 OH O R1,R2,R3 ¼ H, R4 ¼ Prenyl Desmethylxanthohumol Haensel and Schulz (1988) R1,R3 ¼ H; R2 ¼ Me, R4 ¼ Prenyl Xanthogalenol Stevens et al. (2000) 0 R1,R2 ¼ Me; R3 ¼ H, R4 ¼ Prenyl 4 -O-Methylxanthohumol Sun et al. (1989) 0 R1,R2,R3 ¼ H, R4 ¼ Geranyl 3 -Geranylchalconaringenin Stevens et al. (1997) 0 0 R1,R2 ¼ H; R3,R4 ¼ Prenyl 3 ,5 -Diprenylchalconaringenin Stevens et al. (2000) 0 R1 ¼ Me, R2 ¼ H; R3,R4 ¼ Prenyl 5 -Prenylxanthohumol Stevens et al.