Extracellular laccase from Pleurotus ostreatus strain EM-1: thermal stability and response to metal ions

Abstract

The metal ion response profile and thermal stability of extracellular laccase from Pleurotus ostreatus strain EM-1, which is widely cultivated in Ghana, were investigated to provide information essential for the establishment of laccase-based applications in the country. P. ostreatus (Jacq. ex. fr) Kummer strain EM-1 was cultivated on a mixture of Triplochiton scleroxylon (wawa) sawdust, rice bran and lime. Extracellular laccase was isolated from spent sawdust four to six days after the appearance of mushroom pinheads and subjected to ammonium sulphate precipitation and gel filtration using Sephadex G-75. Laccase activity was assayed spectrophotometrically at 468 nm using 2,6-dimethoxyphenol in Mcilvaine’s citrate-phosphate buffer, pH 5.0. Two metal ions, Cu2+ and Mn2+, stimulated P. ostreatus strain EM-1 laccase activity. Cu2+ caused a maximal stimulatory effect of 324.4%, while Mn2+ exerted a more moderate stimulatory effect of 180.5%. Magnesium ions had no effect on the activity of the enzyme. Activity decreased by 77.4% after 20 min of incubation at 50°C. During seven days of storage at either 4 or -20°C, laccase activity decreased by 87.7-88.8%. This rate of deactivation was reduced to 28.1-32.8% over the same period when 20 mM CuSO4 was added to the enzyme prior to storage. The findings suggest that P. ostreatus strain EM-1 laccase would not be a suitable biocatalyst for high temperature processes. Furthermore, copper and to a lesser extent, manganese can be used as stimulatory additives during P. ostreatus strain EM-1 laccase-catalyzed processes at ambient temperature and for short-term storage of the enzyme