Enzymatic characterization of apple (Malus. x Domestica)

Abstract

Secondary metabolites play a major role as quality factors in horticultural products by significantly contributing to plant health, optical attributes, sensory attributes and health beneficial effects for the consumer. Dihydrochalcones, a rare class of secondary metabolites, which is, however present in particularly high amounts in apple (Malus domestica. In apple, phloridzin (phloretin 2'-O-glucoside) is prevalent representing more than 90% of the soluble phenolic compounds, but small amounts of 3-hydroxyphloretin and 3-hydroxyphloridzin are also constitutively present. For the systematic investigation, sufficiently comprehensive knowledge of the underlying pathway is yet lacking. Two types of enzymes could catalyze the reaction, polyphenol oxidases (PPOs) and/or cytochrome P450 dependent monooxygenases. To test a possible involvement of the F3'H of apple in the formation of constitutive 3-hydroxydihydrochalcones, cDNA clones of the two types of F3'Hs present in the apple was isolated and heterologously expressed in yeast. Despite showing high F3'H enzyme activity with various flavonoid substrates, hydroxylation of dihydrochalcones was not observed at standard conditions, indicating that F3'H is not part of the dihydrochalcone pathway.