OXIDATION OF DIBENZO-P-DIOXIN BY LIGNIN PEROXIDASE FROM THE BASIDIOMYCETE PHANEROCHAETE-CHRYSOSPORIUM

Dibenzo-p-dioxin (I) was rapidly degraded in ligninolytic cultures of the basidiomycete Phanerochaete chrysosporium. Lignin peroxidase (LiP) oxidized I to generate the following products: catechol(V), dibenzo-p-dioxin-2,3-quinone (VIII), 2-hydroxy-5-(2-hydroxyphenoxy)-1,4-benzoquinone (IX), 4,5-dihydroxy-1,2-benzoquinone (X), 2-(2-hydroxyphenoxy)-1,4-benzoquinone (XI), 4-hydroxy-1,2-benzoquinone (XII), and 1,2-benzoquinone (XIII). Identical products were formed when the reaction was conducted under argon. No incorporation of O-18 into products was observed when the reaction was conducted under O-18(2). Oxidation of I in (H2O)-O-18 resulted in incorporation of two atoms of O-18 into the quinone VIII. Nonenzymatic hydrolysis of the quinone (VIII) yielded catechol (V), IX and X. Hydrolysis of VIII in (H2O)-O-18 resulted in incorporation of O-18 atoms into IX and X, whereas no incorporation of O-18 atoms into V was observed. These results are explained by mechanisms involving the one-electron oxidation of I by LiP to produce the corresponding cation radical. Nucleophilic attack of water on the cation radical generates a 2-hydroxydibenzo-p-dioxin radical, which is oxidized to a delocalized cation. The attack of water at position C-4a of the 2-hydroxydibenzo-p-dioxin cation, followed by oxidation and C-O-C bond cleavage, lead to formation of the quinone (XI), which undergoes 1,4-addition of water and cleavage of the second C-O-C bond to generate V and XII. Similarly, the attack of water on C-3 of the delocalized cation and subsequent oxidation generates the quinone VIII, which undergoes nonenzymatic 1,4-addition of water, followed by C-O-C bond cleavage to generate IX. IX also undergoes a water-addition reaction followed by C-O-C bond cleavage to generate V and X. Alternatively, the quinones IX and XI could undergo enzymatic oxidation of phenolic functions to generate their corresponding phenoxy radicals, which would be in resonance with carbon-centered radicals. Oxidation of these radicals to cations, with subsequent water attack and C-O-C bond cleavage, would yield 1,2-benzoquinone (XIII) and 2,5-dihydroxy-1,4-benzoquinone (XIV) from IX, and XIII and 2-hydroxy-1,4-benzoquinone (XV) from XI, respectively.