Aryl-alcohol oxidase and lignin peroxidase from the white-rot fungus Bjerkandera adusta
Both aryl-alcohol oxidase (AAO) and lignin peroxidase (LiP) have been purified from ligninolytic cultures of the white-rot fungus Bjerkandera adusta. In contrast, the well-studied lignin peroxidase producing white-rot fungus Phanerochaete chrysosporium does not produce AAO. The reactivity of these enzymes in the oxidation of 3,4-dimethoxybenzyl alcohol (veratryl alcohol) and the related lignin model compounds 3,4-dimethoxyphenyl acetic acid (homoveratric acid) and 1-(3,4-dimethoxyphenyl)-2-phenylethanol (α-benzyl veratryl alcohol), a β-1 model, was investigated. In the AAO-catalyzed oxidation of veratryl alcohol, veratraldehyde is isolated as the sole product. Both homoveratric acid and α-benzyl veratryl alcohol are not substrates for the AAO. In contrast, in the oxidation of veratryl alcohol by both the LiP from B. adusta and the LiP from P. chrysosporium veratraldehyde is isolated as the major product. In addition, however, several minor products (three quinones and three ring-opened products), all resulting from one-electron oxidation of the aromatic ring, could be detected. Veratraldehyde is also the major product in the oxidation of both homoveratric acid and α-benzyl veratryl alcohol by the LiPs from both fungi. Monitoring veratraldehyde formation in the veratryl alcohol oxidation is a widely used assay method for LiP activity. However, the presence of AAO or other enzymes oxidizing veratryl alcohol via direct side-chain oxidation might lead to erroneous results, thus it is suggested that additional assay methods should be developed, with preferably direct observation of aromatic ring oxidation products.