Versatile peroxidase (VP) from the white rot fungus Bjerkandera fumosa was isolated and purified by ion exchange and gel filtration chromatography. Its primary catalytic activity in oxidation of Mn(II) was studied in aqueous solutions in the presence of varying concentrations (up to 8 M) of acetonitrile (MeCN), dimethylsulfoxide (DMSO), ethanol, and n-propanol. The observed maximum reaction rate values decreased with the addition of organic solvents in the order: MeCN < n-propanol < DMSO < ethanol. The observed Michaelis constant values decreased in the order: DMSO < MeCN < n-propanol. This lead to various shapes of the specific activity constant values (k(s) = V-max/K-M) of VP in oxidation of Mn(II) in solutions of different organic solvents. K-M values of typical organic substrates of enzymatic reactions increase with addition of organic solvents, due to unfavorable partitioning of substrates between the medium and the active center of the enzyme. In case of inorganic ions, the decrease of the medium polarity caused by addition of the organic solvent should lead to the enhancement of substrate binding by the enzyme due to the variation in the activity coefficient/solubility of the substrates in the studied systems, and therefore to the decrease of K-M values. The results of our experiments confirm this expectation. To further support this thesis, the effect of ethanol on oxidation of 2,6-dimethoxyphenol catalyzed by VP in the presence of Mn(II) was also studied. The observed V-max values were exponentially decreasing with the solvent concentration, while the K-M versus ethanol concentration curve passed through a minimum. The later effect could be described by two exponential relationships (the one decreasing for Mn(II) and the other one increasing for 2,6-dimethoxyphenol). (c) 2006 Elsevier B.V. All rights reserved.