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 ox idized p-coumaric, caffeic and ferulic acids and slowly oxidized sinap ic acid. The oxidation of sinapic acid was greatly enhanced in the pre sence of p-coumaric, caffeic or ferulic acid. Ascorbate (20 mu M) inhi bited the oxidation of ferulic acid by about 70% and completely inhibi ted the oxidation of p-coumaric and ferulic acids. The cell wall fract ion was capable of binding ferulic and sinapic acids but not caffeic a cid. p-Coumaric acid bound only slightly to cell walls. The oxidation of p-coumaric and ferulic acids by KCI-washed cell walls was inhibited by about 60% and 10%, respectively, by 20 mu M ascorbate, but the oxi dation of caffeic acid was completely inhibited by ascorbate at less t han 20 mu M. The oxidation of derivatives of hydroxycinnamic acid by p eroxidases released from cell walls by washing with 1M KCl was complet ely inhibited by ascorbate. These results suggest that the inhibition by ascorbate depends on the substituent group of the phenyl ring of th e derivatives of hydroxycinnamic acid when the oxidation reaction is c atalyzed by cell wall-bound peroxidases and that the oxidation of sina pic acid is mediated by phenoxyl radicals of derivatives of hydroxycin namic acid other than sinapic acid.