Oxidation of lignin in eucalyptus kraft pulp by manganese peroxidase from Bjerkandera sp strain BOS55

The white rot fungus Bjerkandera sp. strain BOS55 was shown in previous stu dies to cause high levels of kraft pulp bleaching and delignification under culture conditions in which manganese peroxidase (MnP) occurs as the domin ant oxidative enzyme. In this study, the MnP of Bjerkandera was isolated an d tested in vitro with eucalyptus oxygen-delignified kraft pulp (ODKP) base d on measuring the reduction in kappa number as an indicator of lignin oxid ation. The MnP preparation applied at 60 U/g pulp for 6 h caused a signific ant decrease of 11-13% in the kappa number in the ODKP under optimal condit ions compared to parallel-incubated controls lacking enzyme. The effects of MnP dosage, Mn2+ concentration, organic acid buffer selection, pH and H2O2 addition were evaluated. The optimal Mn2+ concentration range for lignin o xidation in ODKP was 100-500 muM. In the presence of low oxalate concentrat ions (0.3-2 mM), the Bjerkandera, a MnP also significantly reduced the kapp a number of ODKP by 6% without any Mn. This observation is in agreement wit h the fact that purified Bjerkandera MnP has Mn-independent activities. Und er incubation conditions with added Mn2+, buffers composed of metal-complex ing organic acids provided two-fold better kappa number reductions compared to the inert acetic acid. The optimal H2O2 dosage was found to be 0.017 mu mol/min ml when added as semicontinuous pulses (every 30 min) or 0.2 mu mo l/min ml when generated continuously by glucose oxidase. Excess H2O2 caused severe inactivation of MnP during the incubations. Factors that improved t he turnover of the enzyme, such as Mn2+ and metal-chelating acids, stabiliz ed MnP against rapid inactivation. (C) 2001 Elsevier Science Ltd. All right s reserved.