FORMATION OF AROMATIC INTERMEDIATES IN THE BIOSYNTHESIS OF BITTER ACIDS IN HUMULUS-LUPULUS

A new hypothesis concerning the formation of aromatic intermediates in the biosynthesis of hop bitter acids was tested. Two phloroglucinol derivatives, 2-(3-methylbutanoyl)-1,3,5-benzenetriol (phlorisovalerophenone = PIVP) and 2-(2-methylpropanoyl)-1,3,5-benzenetriol (phlorisobutyrophenone = PIBP), were detected in hop extracts. The formation of these compounds was shown to occur in vitro in hop protein extracts using the precursors malonyl-CoA plus isovaleryl-CoA or isobutyryl-CoA, respectively. We conclude that PIVP and PIBP are probably the first aromatic intermediates formed in the biosynthesis of humulone/lupulone and cohumulone/colupulone, respectively. We suggest that the reactions are catalysed by a chalcone synthase-like enzyme. The enzyme chalcone synthase and its activity were found to be present in hop protein extracts. The formation of naringenin, PIVP and PIBP in various hop protein extracts obtained during the development from flower bud to ripe cone was compared.