Veratryl alcohol oxidase from Pleurotus ostreatus participates in lignin biodegradation and prevents polymerization of laccase-oxidized substrates

Oxidative enzymes (laccases and peroxidases) isolated from the culture media of different fungi are involved in the basic mechanism of ligninolysis via radical intermediates. However, experiments aimed at reproducing natural biodegradation in vitro have been unsuccessful so far since the single biocatalysts alone are not able to solubilize lignins because of the simultaneous recondensation of these intermediates. FAD oxidases can prevent this side reaction in lignin depolymerization by reducing quinonoids and radical compounds. This study investigates the possible role of a laccase and a FAD-dependent aryl alcohol oxidase (veratryl alcohol oxidase, VAO) excreted by the basidiomycete Pleurotus ostreatus. In fact, we found that VAO is able to reduce synthetic quinones, laccase-generated quinonoids, and phenoxy radicals with concomitant oxidation of veratryl alcohol to veratryl aldehyde. This cooperative action of laccase and VAO also prevented the polymerization of phenolic compounds and reduced the molecular weight of soluble lignosulfonates to a significant extent.