The white rot fungus (WRF) Pleurotus ostreatus produced manganese peroxidase (MnP) and mangarese-independent peroxidase (MIP) activities during solid state fermentation of wheat straw, a natural lignocellulosic substrate. Most of the sulfonphthalein (SP) dyes were decolorized by MnP at pH 4.0. The higher K-m for meta-cresol purple (40 muM) and lower K-m for ortho-cresol red (26 muM) for MnP activities explained the preference for the position of methyl group at ortho than at meta on chromophore. Bromophenol blue decolorizing activity was higher at pH 3.5 and decreased as the concentration of Mn-II was increased. SP-decolorizing activity was associated not only with MnP but also with MIP Additional bromine group along with the methyl group on SP chromophores decreases the rate of decolorization. Bromination of sulfonphthalein chromophore makes them the poorer substrate for MO. This is evident from the higher K-m for bromocresol green (117 muM) when compared to bromocresol purple (36 muM) and bromophenol blue (78 muM). The order of preference for the SP dyes as substrate for the MnP-catalyzed decolorizing activity is phenol red > ortho-cresol red > meta-cresol purple > bromophenol red > bromocresol purple > bromophenol blue > bromocresol green and the order of preference for the SP dyes as substrate for the MIP-catalyzed decolorizing activity is bromocresol green > bromophenol blue > bromocresol purple > bromophenol red > meta-cresol purple > ortho-cresol red > phenol red. Inhibition of PR decolorizing activity by NaN3 provided the evidence of decolorizing activity as an oxidative process. (C) 2004 Elsevier Inc. All rights reserved.