Veratryl alcohol oxidases from the lignin-degrading basidiomycete Pleurotus sajor-caju

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Abstract
The basidiomycete Pleurotus sajor-caju mineralizes ring-14C-labelled lignin (dehydrogenative polymer) when grown in mycological broth. Under these conditions, two veratryl alcohol oxidase (VAO) enzymes were found in the culture medium. They oxidized a number of aromatic alcohols to aldehydes and reduced O2 to H2O2. The enzymes were purified by ion-exchange and gel-permeation chromatography. The final step of purification on Mono Q resolved the activity into two peaks (VAO I and VAO II). Both enzymes had the same Mr, approx. 71,000, but their isoelectric points differed slightly, 3.8 for VAO I and 4.0 for VAO II. Their amino acid compositions were similar except for aspartic acid/asparagine and glycine. Both enzymes are glycoproteins and contain flavin prosthetic groups. Their pH optima were around 5, and kinetic constants and specificities were similar. 4-Methoxybenzyl alcohol was oxidized the most rapidly, followed by veratryl alcohol. Not all aromatic alcohols were oxidized, neither were non-aromatic alcohols. Cinnamyl alcohol was oxidized at the gamma position. The VAO enzymes thus represent a significantly different route for veratryl alcohol oxidation from that catalysed by the previously found lignin peroxidases from Phanerochaete chrysosporium. The role of the oxidases in biodegradation might be to produce H2O2 during oxidation of lignin fragments.