BIOSYNTHESIS OF P-HYDROXYPHENYLBUTAN-2-ONE IN RASPBERRY FRUITS AND TISSUE-CULTURES

p-Hydroxyphenylbutan-2-one (pHPB), the raspberry ketone, is responsible for the characteristic aroma of raspberries. The compound accumulates rapidly during the later maturation stages of the berries. The synthesis and accumulation of pHPB correlates with that of anthocyanin and soluble solids (Brix). pHPB is synthesized in cell-free extracts of fruits and tissue cultures from p-coumaryl-CoA and malonyl-CoA in a manner similar to the synthesis of chalcones and stilbenes. The specific biosynthetic pathway for pHPB formation deviates from the general phenylpropanoid pathway at the p-coumaryl-CoA stage and it is composed of two enzymes. The first enzyme is the p-hydroxyphenylbut-3-ene-2-one synthase (pHPB-3-ene-2-one synthase) that forms p-hydroxyphenylbut-3-ene-2-one by the condensation of malonyl-CoA with p-coumaryl-CoA. The second enzyme, p-hydroxyphenylbut-3-ene-2-one reductase (pHPB-3-ene-2-one reductase), reduces the p-hydroxyphenylbut-3-ene-2-one to p-hydroxyphenylbutan-2-one, the raspberry ketone. We detected the activity of both enzymes in crude extracts from raspberry fruits and their tissue cultures, and identified their reaction products by HPLC, crystallization to constant radioactivity and by GC-MS.