Demethoxylation of [O14CH3]-labelled lignin model compounds by the brown-rot fungi Gloeophyllum trabeum and Poria (Postia) placenta.

Demethoxylation reactions in the cultures of the brown-rot fungi Gloeophyllum trabeum and Poria placenta were studied by determining the evolution of (14)CO(2) from a non-phenolic lignin model, beta-O-4 dimer, [O(14)CH(3)]-labelled at position 4 in the A ring (model I), and from [O(14)CH(3)]-labelled vanillic acid (model II). The fungi were grown under oxygen or air atmosphere on an agar medium with or without spruce sapwood blocks. The dimeric model (I) was impregnated onto agar or wood block in cultures to clarify the possible effect of wood as growth substrate. In the case of vanillic acid (model II), birch wood was used. The effect of supplemented nutrient nitrogen (2 mM N) and glucose (0.1 or 1.0% w/v) on demethoxylation was also studied. G. trabeum enhanced the production of (14)CO(2) from the dimer in the presence of spruce wood blocks. It released (14)CO(2) from the methoxyl groups giving 30-60% of the applied activity in 8 weeks. P. placenta produced almost 30% (14)CO(2 )from vanillic acid (model II) in 9 weeks under oxygen, but from the methoxyl group of the dimer only 3% of (14)CO(2) was evolved in 4 weeks. The biomasses determined as ergosterol assay showed variation from 14 to 226 microg g(-1) dry weight of agar, and 2 to 9 microg g(-1 )of wood, but they did not correlate with the production of (14)CO(2). The results indicate that these brown-rot fungi possess different mechanisms for demethoxylation.

Wood stimulates the demethoxylation of [O14CH3]-labeled lignin model compounds by the white-rot fungi Phanerochaete chrysosporium and Phlebia radiata.

Mineralization of polymeric wood lignin and its substructures is a result of complex reactions involving oxidizing and reducing enzymes and radicals. The degradation of methoxyl groups is an essential part of this process. The presence of wood greatly stimulates the demethoxylation of a non-phenolic lignin model compound (a [O(14)CH(3)]-labeled beta-O-4 dimer) by the lignin-degrading white-rot fungi Phlebia radiata and Phanerochaete chrysosporium. When grown on wood, both fungi produced up to 47 and 40% (14)CO(2) of the applied (14)C activity, respectively, under air and oxygen in 8 weeks. Without wood, the demethoxylation of the dimer by both fungi was lower, varying between 0.5 and 35%. Addition of nutrient nitrogen together with glucose decreased demethoxylation when the fungi were grown on spruce wood under air. Because the evolution of (14)CO(2) in the absence of wood was poor, the fungi may have preferably used wood as a carbon and nitrogen source. The amount of fungal mycelium, as determined by the ergosterol assay, did not show connection to demethoxylation. P. radiata also showed a high demethoxylation of [O(14)CH(3)]-labeled vanillic acid in the presence of birch wood. The degradation of lignin and lignin-related substances should be studied in the presence of wood, the natural substrate for white-rot fungi.