A Possible Mechanism for the Oxidation of Sinapyl Alcohol by Peroxidase-Dependent Reactions in the Apoplast: Enhancement of the Oxidation by Hydroxycinnamic Acids and Components of the Apoplast

Apoplastic peroxidase isoenzymes from stems of Nicotiana tabacum rapidly oxidized sinapic acid and sinapyl alcohol, in addition to 4-coumaric acid, ferulic acid and coniferyl alcohol. By contrast, the peroxidase isoenzymes from stems of Vigna angularis oxidized sinapic acid and sinapyl alcohol quite slowly but rapidly oxidized compounds with a 4-hydroxyphenyl or a guaiacyl group. However, the oxidation of sinapyl alcohol was greatly enhanced by 4-coumaric acid, ferulic acid and an ester of ferulic acid. Intercellular washing fluid of V. angularis, which contained apoplastic components, also enhanced the oxidation of sinapyl alcohol. Based on these results, a possible mechanism for the oxidation of sinapyl alcohol is discussed on the assumption that the biosynthesis of lignin proceeds mainly via peroxidases which cannot oxidize sinapyl alcohol in V. angularis.