Anisaldehyde and Veratraldehyde Acting as Redox Cycling Agents for H2O2 Production by Pleurotus eryngii

The existence of a redox cycle leading to the production of hydrogen peroxide (H₂O₂) in the white rot fungus Pleurotus eryngii has been confirmed by incubations of 10-day-old mycelium with veratryl (3,4-dimethoxybenzyl) and anisyl (4-methoxybenzyl) compounds (alcohols, aldehydes, and acids). Veratraldehyde and anisaldehyde were reduced by aryl-alcohol dehydrogenase to their corresponding alcohols, which were oxidized by aryl-alcohol oxidase, producing H₂O₂. Veratric and anisic acids were incorporated into the cycle after their reduction, which was catalyzed by aryl-aldehyde dehydrogenase. With the use of different initial concentrations of either veratryl alcohol, veratraldehyde, or veratric acid (0.5 to 4.0 mM), around 94% of veratraldehyde and 3% of veratryl alcohol (compared with initial concentrations) and trace amounts of veratric acid were found when equilibrium between reductive and oxidative activities had been reached, regardless of the initial compound used. At concentrations higher than 1 mM, veratric acid was not transformed, and at 1.0 mM, it produced a negative effect on the activities of aryl-alcohol oxidase and both dehydrogenases. H₂O₂ levels were proportional to the initial concentrations of veratryl compounds (around 0.5%), and an equilibrium between aryl-alcohol oxidase and an unknown H₂O₂-reducing system kept these levels steady. On the other hand, the concomitant production of the three above-mentioned enzymes during the active growth phase of the fungus was demonstrated. Finally, the possibility that anisaldehyde is the metabolite produced by P. eryngii for the maintenance of this redox cycle is discussed.