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 d erivatives, 2-(3-methylbutanoyl)-1,3,5-benzenetriol (phlorisovalerophe none = PIVP) and 2-(2-methylpropanoyl)-1,3,5-benzenetriol (phlorisobut yrophenone = PIBP), were detected in hop extracts. The formation of th ese compounds was shown to occur in vitro in hop protein extracts usin g the precursors malonyl-CoA plus isovaleryl-CoA or isobutyryl-CoA, re spectively. We conclude that PIVP and PIBP are probably the first arom atic intermediates formed in the biosynthesis of humulone/lupulone and cohumulone/colupulone, respectively. We suggest that the reactions ar e catalysed by a chalcone synthase-like enzyme. The enzyme chalcone sy nthase and its activity were found to be present in hop protein extrac ts. The formation of naringenin, PIVP and PIBP in various hop protein extracts obtained during the development from flower bud to ripe cone was compared.