Quinone redox cycling in the ligninolytic fungus Pleurotus eryngii leading to extracellular production of superoxide anion radical

Quinone redox cycling is generally known as an intracellular process that implies the reduction of quinones (Q) into semiquinones (Q<(.)over bar>) or hydroquinones (QH(2)), which autoxidize reducing oxygen to superoxide anion radical (Oi) We demonstrate here for the first time the existence of quinone redox cycling in a ligninolytic fungus, Pleurotus eryngii, showing two particularities: extracellular production of O-2<(.)over bar> and involvement of ligninolytic enzymes. Experiments were performed with P. eryngii cultures, showing laccase activity, and four quinones: 1,4-benzoquinone (BQ), 2-methyl-1,4-benzoquinone (MeBQ), 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ), and 2-methyl-1,4-naphthoquinone (menadione, MD). The overall process consisted of cell-bound divalent reduction of quinones, followed by extracellular laccase-mediated oxidation of hydroquinones into semiquinones, which autoxidized to a certain extent producing O-2<(.)over bar> (at the pH values of natural degradation of lignin, some autoxidation of hydroquinones was observed only with DQH(2) and MDH(2)). The existence of a redox cyclic system involving quinones was evidenced by determining the chemical state of quinones along incubation under several conditions (either different O-2 concentrations and pH values or laccase amounts). Thus, QH(2)/Q ratios at system equilibrium decreased as either pH values and oxygen concentration (allowing hydroquinones autoxidation) or the amount of laccase increased. Once the cyclic nature of the system was demonstrated, special attention was paid to the production of Oi during hydroquinone oxidation. Except in the case of B&H2, production of O-2<(.)over bar> was found in samples containing hydroquinones and laccase. By the use of agents promoting the autoxidation of semiquinones (superoxide dismutase and Mn2+), production of O-2<(.)over bar> during oxidation of BQH(2) could finally be demonstrated. (C) 1997 Academic Press.