The complexity of oxidative cross-linking of phenylpropanoids — evidence from an in vitro model system

Abstract

The development of an in vitro model system to study the process of dimerization of hydroxycinnamic acids is reported. Model compounds ferulic acid (FA) or methyl ferulate were reacted with H₂O₂ in the presence of horseradish peroxidase (HRP) or peroxidases from Lupinus albus L. apoplastic fluids. Following solid-phase extraction, the reaction products were analysed using gas chromatography-mass spectrometry. Major analytical and biochemical parameters of model reactions were determined and optimized. Six ferulic dimers were identified and quantified.

With the use of this model system we found methyl ferulate to be a better substrate for lupin and horseradish peroxidases than FA. Use of various peroxidases did not influence the qualitative composition of reaction products although it affected the rate of substrate utilization and the quantitative output of reactions. Various isoforms of lupin apoplastic peroxidases revealed differentiated specificity towards hydroxycinnamic acids or their methyl esters. The potential of isoflavonoids (major phenolic components in white lupin cell walls) to influence peroxidase-catalysed formation of ferulic dehydrodimers was also tested. Genistein (5,7,4'-trihydroxyisoflavone) and genistin (genistein-7-O-β-glucoside) did not affect the qualitative composition of the reaction products. However, genistein inhibited the rate of ferulic substrate oxidation, while genistin showed the opposite effect, stimulating both utilization of ferulic substrate by HRP and subsequent polymerization and / or degradation of dehydrodimers formed. To our knowledge, this is the first indirect evidence that isoflavonoids might play a regulatory role in the oxidative formation of intermolecular cross-links in cell walls.