Oxidation of polycyclic aromatic hydrocarbons and dibenzo[p]-dioxins by Phanerochaete chrysosporium ligninase

Abstract

The lignin peroxidase (ligninase) of Phanerochaete chrysosporium catalyzes the oxidation of a variety of lignin-related compounds. Here we report that this enzyme also catalyzes the oxidation of certain aromatic pollutants and compounds related to them, including polycyclic aromatic hydrocarbons with ionization potentials less than or equal to approximately 7.55 eV. This result demonstrates that the H2O2-oxidized states of lignin peroxidase are more oxidizing than the analogous states of classical peroxidases. Experiments with pyrene as the substrate showed that pyrene-1,6-dione and pyrene-1,8-dione are the major oxidation products (84% of total as determined by high performance liquid chromatography), and gas chromatography/mass spectrometry analysis of ligninase-catalyzed pyrene oxidations done in the presence of H2(18)O showed that the quinone oxygens come from water. We found that whole cultures of P. chrysosporium also transiently oxidize pyrene to these quinones. Experiments with dibenzo[p]dioxin and 2-chlorodibenzo[p]dioxin showed that they are also substrates for ligninase. The immediate product of dibenzo[p]dioxin oxidation is the dibenzo[p]dioxin cation radical, which was observed in enzymatic reactions by its electron spin resonance and visible absorption spectra. The cation radical mechanism of ligninase thus applies not only to lignin, but also to other environmentally significant aromatics.