molecules Review Flavonoids as Phytoestrogenic Components of Hops and Beer Tomasz Tronina * , Jarosław Popło ´nski and Agnieszka Bartma ´nska Department of Chemistry, Wrocław University of Environmeuintal and Life Sciences, Norwida 25, 50-375 Wrocław, Poland; [email protected] (J.P.); [email protected] (A.B.) * Correspondence: [email protected]; Tel.: +48-71-320-50-19 Academic Editor: Celestino Santos-Buelga Received: 30 August 2020; Accepted: 10 September 2020; Published: 14 September 2020 Abstract: The value of hops (Humulus lupulus L.) in beer production has been undisputed for centuries. Hops is rich in humulones and lupulones which gives the characteristic aroma and bitter taste, and preserves this golden drink against growing bacteria and molds. Besides α- and β-acids, the lupulin glands of hop cones excrete prenylated flavonoids, which exhibit a broad spectrum of biological activities and therefore has therapeutic potential in humans. Recently, interest in hops was raised due to hop prenylated flavanones which show extraordinary estrogen activities. The strongest known phytoestrogen so far is 8-prenylnaringenin (8-PN), which along with 6-prenylanaringenin (6-PN), 6,8-diprenylnaringenin (6,8-DPN) and 8-geranylnaringenin (8-GN) are fundamental for the potent estrogen activity of hops. This review provides insight into the unusual hop phytoestrogens and shows numerous health benefits associated with their wide spectrum of biological activities including estrogenic, anticancer, neuropreventive, antinflamatory, and antimicrobial properties, which were intensively studied, and potential applications of these compounds such as, as an alternative to hormone replacement therapy (HRT). Keywords: phytoestrogens; prenylflavonoids; biological activities; hop; beer 1. Hop Hop (Humulus lupulus L.) is a vine belonging to the genus Humulus from the family Cannabaceae [1]. Hops are dioecious perennials found in the northern hemisphere and reach a height of 7–8 m. Plants are cultivated in North America mainly in Idaho, Oregon, and Washington and in Europe in Germany, Great Britain, Poland, and the Czech Republic. In Asia, cultivation takes place in some areas of China, and to a limited extent in Japan, and in the southern hemisphere, in some regions of Australia and New Zealand [2]. The historic use of the female inflorescences of Humulus lupulus (hop) is interesting because its technical properties—as the source of bitter taste and preservation of beer—were discovered in the Middle Ages. According to Wiesner (1883), so-called herbal beers were produced in the years 1300–1500 [2]. In traditional medicine, hops have long been used to treat sleep disorders, as a stomach remedy, and as an antibacterial and antifungal agent [3]. Humulus lupulus attracted special attention because it contains many dietary phytochemicals that can be used for medicinal purposes, due to their antiseptic, (an)aphrodisiac, anticancer, antiplatelet, antidiuretic, antiinflammatory and sedative properties [4]. Hops are rich in phenolic compounds, mostly flavonoids, which are secondary metabolites (Figure1). Mass spectrometry analysis shows that the plant contains about 14.4% phenolic acids, flavonoids, proanthocyanidins, prenylated chalcones and flavanones as well as catechins [5]. In addition, malt accounts for 70–80% of all polyphenolic compounds found in beer [6]. Molecules 2020, 25, 4201; doi:10.3390/molecules25184201 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 21 flavonoids, proanthocyanidins, prenylated chalcones and flavanones as well as catechins [5]. In addition, malt accounts for 70–80% of all polyphenolic compounds found in beer [6]. 1.1. Hop Prenylflavonoids Flavonoids are phenolic derivatives with a flavan or chalcone flavonoid skeleton and have many beneficial effects on human health [7,8]. This is especially true of prenylflavonoids, which have so far beenMolecules identified2020, 25, 4201 in 37 of plant genera, mainly in the families of Leguminosae, Moraceae, Guttiferae2 of 21, Umbelliferae, Rutaceae and Cannabaceae [9]. Figure 1. 1. BiosynthesisBiosynthesis pathways pathways of ofseveral several hop hop flavon flavonoids.oids. PAL: PAL: Phenylalanine Phenylalanine ammonia-lyase; ammonia-lyase; C4H cinnamateC4H cinnamate 4-hydroxylase, 4-hydroxylase, 4CL 4-coumarate-CoA 4CL 4-coumarate-CoA ligase, AAC ligase, acetyl-CoA AAC carboxylase, acetyl-CoA CS carboxylase, chalcone synthase,CS chalcone HIPT1 synthase, Humulus HIPT1 lupulusHumulus prenyltansferase-1,CHI lupulus prenyltansferase-1,CHI chalcone isomerase, chalcone isomerase,OMT O- methyltransferase,OMT O-methyltransferase, F3H flavanone F3H flavanone 3-hydroxylase, 3-hydroxylase, FLS flavonol FLS flavonol synthase synthase,, DFR DFR dihydroflavonol dihydroflavonol 4- reductase,4-reductase, LAR LAR leucoanthocyanidin leucoanthocyanidin reductase, reductase, ANS ANS anthocyanidin anthocyanidin syntha synthase,se, ANR anthocyanidin reductase, FMO FMO flavonoid flavonoid 3-monooxygenas 3-monooxygenase,e, CPR CPR cytochrome cytochrome P450 P450 reductase, reductase, F3′5 F3′H0 5flavonoid0H flavonoid 3′5′- hydroxylase.3050-hydroxylase. 1.1. Hop Prenylflavonoids These compounds differ in the position of the prenyl group, the number of attached isoprenoid unitsFlavonoids and modifications are phenolic of the derivatives prenyl moiety, with asuch flavan as cyclization or chalcone and flavonoid hydroxylation skeleton [10]. and have many beneficialPrenylated effects flavonoids on human are health accumulated [7,8]. This in is lupulin especially glands true that of prenylflavonoids,cover bract of female which hop have cones. so Theirfar been common identified precursor in 37 of compound plant genera, xanthohumol mainly in (XN) the families (1) (Figure of Leguminosae 2), a prenylated, Moraceae chalcone,, Guttiferae is the, Umbelliferaeprincipal prenylflavonoid, Rutaceae and Cannabaceae in the female[9 flowers]. of the hop plant Humulus lupulus L. and provides 80– 90% ofThese its total compounds flavonoid di contentffer in the [11]. position of the prenyl group, the number of attached isoprenoid unitsXN and (1 modifications) was extensively of the studied prenyl in moiety, the past such and as is cyclization considered and the hydroxylation most active ingredient [10]. in hop and beer.Prenylated It demonstrated flavonoids chemopreventive are accumulated inactivity lupulin [12], glands inhibiting that cover initiation bract and of female development hop cones. of Theircarcinogenesis common in precursor human body compound [13,14] xanthohumol and therapeuti (XN)c activity, (1) (Figure by inducing2), a prenylated cytotoxic chalcone, effect [13] is and the principalapoptosis prenylflavonoidin the existing tumour in the female cells [15–17]. flowers In of addition, the hop plant it positivelyHumulus affects lupulus glucoseL. and and provides lipid metabolism80–90% of its [18–20], total flavonoid hepatic and content intestinal [11]. metabolism and many others activities [21,22]. Molecules 2020, 25, 4201 3 of 21 Molecules 2020, 25, x FOR PEER REVIEW 3 of 21 Figure 2.2. StructuresStructures of of described described flavonoids: flavonoids: xanthohumol xanthohumol XN XN ( (11),), isoxanthohumol isoxanthohumol IX IX ( (2),), desmethylxanthohumol DMX DMX (3 (),3), 8-prenylnaringenin 8-prenylnaringenin 8-PN 8-PN (4), ( 46-prenylnaringenin), 6-prenylnaringenin 6-PN 6-PN (5), 6,8- (5), 6,8-diprenylnaringenindiprenylnaringenin 6,8-DPN 6,8-DPN (6), (8-geranylnaringenin6), 8-geranylnaringenin 8-GN 8-GN (7). (7). XNDuring (1) wasbeer extensively production studiedhigh temp in theerature past causes and is isomerization considered the XN most (1) toIXN active ingredient(2) which become in hop anda major beer. prenyl It demonstrated flavonoid present chemopreventive in beer; however, activity in [ 12the], inhibitinghydrophobic initiation environment and development of plant cells, of carcinogenesisisomerization of in XN human (1) to body IX (2 [)13 was,14 ]not and observed, therapeutic hence activity, hop’s byprenyflavanones inducing cytotoxic are represented effect [13] and by apoptosisIXN (1), which in the is existingonly about tumour 0.01% cellsof dry [15 matter–17]. In[23]. addition, it positively affects glucose and lipid metabolism8-PN (4 [)18 has–20 been], hepatic identified and intestinal in an amount metabolism of 0.002% and of many dry matter others of activities hops [11], [21 ,as22 ].well as 6-PN (5) whichDuring is present beer production in slightly high higher temperature amounts (0.01%) causes [24]. isomerization Both compounds XN (1) toIXN(4, 5) are (2) produced which become from aDMX major (3) prenyl in small flavonoid quantities present in a non-enzymatic in beer; however, manner in the during hydrophobic the drying, environment storage and of extraction plant cells, of isomerizationhops. of XN (1) to IX (2) was not observed, hence hop’s prenyflavanones are represented by IXN (1), which is only about 0.01% of dry matter [23]. 1.2. Prenylflavonoids8-PN (4) has been in Beer identified in an amount of 0.002% of dry matter of hops [11], as well as 6-PN (5) whichThe is presentuse of high in slightly performance higher amountsliquid chromatograp (0.01%) [24].hy Both and compounds posteriori ultrasonic (4, 5) are producedseparation from has DMXshown ( 3that) in fermentation, small quantities boiling in a non-enzymaticand the amount mannerof hops duringused in thebeer drying, production storage have and a significant extraction ofeffect hops. on the concentration of polyphenols