Chemistry and Biology of Hop Flavonoids

Abstract

The flavonoids of hops include flavonol glycosides, condensed tannins, and prenylflavonoids. The first two flavonoid classes are located intracellularly, while the prenylflavonoids are secreted along with bitter acids and essential oils by the lupulin glands of the inflorescences. The chemistry and biological activities of prenylflavonoids isolated from the lupulin fraction are reviewed. An LC-MS method was developed for quantitation of these phenolics in hops, hop-derived products, and beer. The fate of xanthohumol and related prenylflavonoids during the brewing process was investigated by LC-MS. The partial carryover of the prenylflavonoids from hops into beer was attributed mainly to incomplete extraction and adsorption to insoluble proteins and yeast. Moreover, the prenylchalcone-type flavonoids were largely converted into isomeric prenylflavanones during wort boiling. The biological activities of some of the prenylflavonoids isolated were examined using in vitro techniques. Some of the prenylflavonoids inhibited the growth of breast cancer (MCF-7) cells in a dose-dependent manner. Xanthohumol and related prenylflavonoids also inhibited the cytochrome P450-mediated activation of procarcinogens and induced the activity of the carcinogen-detoxifying enzyme, quinone reductase. The prenylflavonoids were not toxic to normal cells (rat hepatocytes) at the same concentrations. Recent literature reports of biological activities are briefly discussed: inhibition of bone resorption, inhibition of diacylglycerol acyltransferase, and antimicrobial activities.