Veratryl alcohol-mediated indirect oxidation of phenol by lignin peroxidase

Veratryl alcohol (VA) oxidation by lignin peroxidase (LiP) was inhibited by phenol. The enzyme was quickly converted to compound III, an inactive intermediate. However, as soon as VA began to be oxidized, compound II was observed. The lag period before VA oxidation was affected by the concentrations of both phenol and VA. The addition of VA increased the extent of phenol oxidation and the kinetics of phenol oxidation in the presence of VA were similar to those of VA oxidation. Previously it was shown that the VA cation radical (VA^·+) was responsible for the conversion of compound III back to ferric enzyme [D. P. Barr and S. D. Aust (1994) Arch. Biochem. Biophys. 312, 511-515]. Here we observed that the reversion of compound III to active ferric enzyme in the presence of VA was prevented by addition of phenol, suggesting that VA^·+ oxidizes phenol to phenoxyl radical. This hypothesis was also supported by the observation that O₂ consumption during VA oxidation was inhibited by the addition of phenol. All of these results suggested that VA was first oxidized by LiP to VA^·+ and then it oxidized phenol to phenoxyl radical while VA^·+ was reduced back to VA. Activity was lost as compound III accumulated since the VA^·+ was consumed by the oxidation of phenol. After all of the phenol was oxidized, VA^·+ became available to convert compound III back to ferric enzyme.