Inactivation of lignin peroxidase by hydrogen peroxide during the oxidation of phenols

The oxidation of phenols by lignin peroxidase (LiP) was characterized by a rapid decrease in enzyme activity. The initial oxidation rate of phenol decreased at moderately high (i.e., 400 μM) concentrations of H₂O₂. Similar results were obtained with several phenols. However, LiP was not inactivated during the oxidation of veratryl alcohol (VA). The apparent second-order rate constants for the reaction of compound II with phenol and VA at pH 3.5 were very similar, about 2.1 × 10⁴ and 4.7 × 10⁴ M^-1 s^-1, respectively. However, unlike VA, phenols could not convert compound III back to ferric enzyme. The visible absorption spectra of compound II and III were observed during oxidation of VA and phenols, respectively. These results suggest that the inactivation of LiP during the oxidation of phenols was mainly due to the accumulation of compound III, which was attributed to the inability of phenols or phenoxyl radicals to revert compound III to ferric enzyme. All of these results also suggested that the inactivation mechanism of LiP during oxidation of phenols was different from that during oxidation of anisyl alcohol, since anisyl alcohol was neither a substrate of compound II nor could it revert compound III [K. Valli et al. (1990) Biochemistry 29, 8535-8539 and R. S. Koduri and M. Tien (1994) Biochemistry 33, 4225-4230].