Production of manganese peroxidase and organic acids and mineralization ofC-14-labelled lignin (C-14-DHP) during solid-state fermentation of wheat straw with the white rot fungus Nematoloma frowardii

The basidiomycetous fungus Nematoloma frowardii produced manganese peroxida se (MnP) as the predominant ligninolytic enzyme during solid-state fermenta tion (SSF) of wheat straw. The purified enzyme had a molecular mass of 50 k Da and an isoelectric point of 3.2. In addition to MnP, low levels of lacca se and lignin peroxidase were detected. Synthetic C-14-ring-labelled lignin (C-14-DHP) was efficiently degraded during SSF, Approximately 75% of the i nitial radioactivity was released as (CO2)-C-14, while only 6% was associat ed with the residual straw material, including the well-developed fungal bi omass, On the basis of this finding,ve concluded that at least partial extr acellular mineralization of lignin may have occurred. This conclusion was s upported by the fact that we detected high levels of organic acids in the f ermented straw (the maximum concentrations in the water phases of the straw cultures were 45 mM malate, 3.5 mM fumarate, and 10 mM oxalate), which ren dered MnP effective and therefore made partial direct mineralization of lig nin possible. Experiments performed in a cell-free system, which simulated the conditions in the straw cultures, revealed that MnP in fact converted p art of the C-14-DHP to (CO2)-C-14, (which accounted for up to 8% of the ini tial radioactivity added) and C-14-labelled water-soluble products (which a ccounted for 43% of the initial radioactivity) in the presence of natural l evels of organic acids (30 mM malate, 5 mM fumarate).