Laccases of Pleurotus ostreatus observed at different phases of its growth in submerged fermentation: production of a novel laccase isoform

The production of laccases during the lag, exponential and stationary phases of growth of Pleurotus ostreatus in submerged fermentation was evaluated. Laccase activity was positively correlated to the growth of the fungus. The specific growth rate was 0.02 h−1 and the highest amount of dry biomass (7.8 g l−1) was obtained after 480 h of growth. Four laccase isoforms were secreted by the fungus, and tentatively named LI1, LI2, LI3 and LI4. LI2, LI3 and LI4 were produced during the stationary phase (between 408 and 456 h approximately) while LI1 was produced during the lag, exponential and stationary phases of growth. Maximal laccase activity (12 200 U l−1) was observed at the beginning of the stationary phase (at 432 h of growth). LI1 was purified by preparative isoelectric focusing and partially characterized. LI1 had a molecular mass of 43.7 kDa as determined by SDS PAGE, a Km and Vmax of 90 μM and 1.18 ΔAbs min−1 respectively and an isoelectric point of 2.3. LI1 showed activity over a broad range of pH and temperature, which may make it useful in the biodegradation of phenolic compounds present in wastewater from several industrial processes.