Degradation of humic acids by manganese peroxidase from the white-rot fungus Clitocybula dusenii.

The depolymerization of humic acids (HAs) obtained from low-rank coal (lignite) to fulvic acids (FAs) was investigated in a cell-free system (in vitro) using manganese peroxidase (MnP) from the white-rot fungus Clitocybula dusenii b11. MnP was produced in surface cultures of C. dusenii which were induced with manganese (II) ions (Mn2+, 300 microM). The optimum conditions for the action of MnP were determined by varying following parameters of the enzyme assay: i) concentration of Mn2+, ii) concentration of hydrogen peroxide (H2O2), iii) pH value and iv) temperature. Optimum parameters determined were used in subsequent in vitro depolymerization studies of humic acids. For that purpose, following parameters of the reaction mixture were additionally varied: concentration of HAs, concentration of the thiol mediator glutathione (GSH), presence and concentration of organic solvents. As the result, following parameters were found to be optimal for the MnP-catalyzed in vitro depolymerization of HAs into low-molecular weight FAs (MnP activity 0.12 U/ml): 250 micrograms/ml HAs, 1 mM MnCl2, 46 microM/min H2O2 (continuously supplied by glucose oxidase), 600 microM GSH, 4% (v/v) N,N-dimethylformamide (DMF), pH 4.0, and 37 degrees C.