Microsomal transformation of organophosphorus pesticides by white rot fungi

The enzymatic mechanism for the transformation of organophosphorus pesticides (OPPs) by different white-rot fungi strains was studied. With the exception of Ganoderma applanatum 8168, all strains from a collection of 17 different fungi cultures were able to deplete parathion. Three strains showing the highest activities were selected for further studies: Bjerkandera adusta 8258, Pleurotus ostreatus 7989 and Phanerochaete chrysosporium 3641. These strains depleted 50 to 96% of terbufos, azinphos-methyl, phosmet and tribufos after four-days exposure to the pesticides. In order to identify the cellular localization of the transformation activity, the extracellular and microsomal fractions of Pleurotus ostreatus 7989 were evaluated in vitro. While the activities of ligninolytic enzymes (lignin peroxidase, manganese peroxidase and laccase) were detected in the extracellular fraction, no enzymatic modification of any of the five pesticides tested could be found, suggesting the intracellular origin of the transformation activity. In accordance with this observation the microsomal fraction was found able to transform three OPPs with the following rates: 10 mumol mg prot(-1) h(-1) for phosmet, 5.7 mumol mg prot(-1) h(-1) for terbufos, and 2.2 mumol mg prot(-1) h(-1) for azinphos-methyl. The products from these reactions and from the transformation of trichlorfon and malathion, were identified by mass-spectrometry. These results, supported by specific inhibition experiments and the stringent requirement for NADPH during the in vitro assays suggest the involvement of a cytochrome P450.