OXIDATION OF HYDROXYCINNAMIC ACID AND HYDROXYCINNAMYL ALCOHOL DERIVATIVES BY LACCASE AND PEROXIDASE - INTERACTIONS AMONG P-HYDROXYPHENYL, GUAIACYL AND SYRINGYL GROUPS DURING THE OXIDATION REACTIONS

Fungal laccase oxidized derivatives of hydroxycinnamic acid. The rates decreased in the order sinapic acid > ferulic acid greater than or eq ual to p-coumaric acid. The laccase oxidized sinapyl alcohol faster th an coniferyl alcohol. The rates of oxidation of the hydroxycinnamic ac id derivatives by an isoenzyme of peroxidase from horseradish decrease d in the order p-coumaric acid > ferulic acid greater than or equal to sinapic acid. The peroxidase oxidized coniferyl alcohol much faster t han sinapyl alcohol. The laccase and the peroxidase predominantly oxid ized (a) ferulic acid in a reaction mixture that contained p-coumaric acid and ferulic acid, (b) sinapic acid in a mixture of p-coumaric aci d plus sinapic acid, and (c) sinapic acid in a mixture of ferulic acid plus sinapic acid. In a reaction mixture that contained both conifery l and sinapyl alcohols, both fungal laccase and horseradish peroxidase predominantly oxidized sinapyl alcohol. From these results, it is con cluded (1) that the p-hydroxyphenyl radical can oxidize guaiacyl and s yringyl groups and produce their radicals and (2) that the guaiacyl ra dical can oxidize the syringyl group under formation of its radical; a nd that (3) in both cases the reverse reactions are very slow.