Effects of Ascorbate on the Oxidation of Derivatives of Hydroxycinnamic Acid and the Mechanism of Oxidation of Sinapic Acid by Cell Wall-Bound Peroxidases

A cell wall fraction isolated from epicotyls of Vigna angularis, which contained both ionically and covalently bound peroxidases, rapidly oxidized p-coumaric, caffeic and ferulic acids and slowly oxidized sinapic acid. The oxidation of sinapic acid was greatly enhanced in the presence of p-coumaric, caffeic or ferulic acid. Ascorbate (20 μM) inhibited the oxidation of ferulic acid by about 70% and completely inhibited the oxidation of p-coumaric and ferulic acids. The cell wall fraction was capable of binding ferulic and sinapic acids but not caffeic acid. p-Coumaric acid bound only slightly to cell walls. The oxidation of p-coumaric and ferulic acids by KCl-washed cell walls was inhibited by about 60% and 10%, respectively, by 20 μM ascorbate, but the oxidation of caffeic acid was completely inhibited by ascorbate at less than 20 μM. The oxidation of derivatives of hydroxycinnamic acid by peroxidases released from cell walls by washing with 1 M KCl was completely inhibited by ascorbate. These results suggest that the inhibition by ascorbate depends on the substituent group of the phenyl ring of the derivatives of hydroxycinnamic acid when the oxidation reaction is catalyzed by cell wall-bound peroxidases and that the oxidation of sinapic acid is mediated by phenoxyl radicals of derivatives of hydroxycinnamic acid other than sinapic acid.