Evaluation of SOS-inducing activity of organic extracts from tributaries of the Yodo River in Kyoto was examined with a highly sensitive umu test system using an O -acetyltransferase(O -ATase)-overexpressing strain Salmonella typhimurium NM2009 and an O-ATase-defective strain NM2000. Organic extracts in river water were collected by the XAD-2 resin column method and the blue rayon hanging method at 4 locations throughout the year. Both XAD-2 resin and blue rayon extracts collected at downstream locations from wastewater treatment plants showed higher SOS-inducing activity than those at the upsream location in the absence and the presence of S9 mix with NM2009. It was also shown that the constituents contributing SOS-inducing activity were different between XAD-2 extract and blue rayon extract. No or weak responses of SOS-inducing activity were observed with an O -ATase-defective strain NM2000 among all extracts performed. These results indicate that a highly sensitive umu test using an O-ATase-overexpressing strain and an O-ATase-defective strain proved to be an excellent tool for the monitoring of genotoxicity attributable to trace amounts of genotoxicants such as direct-acting nitroarenes and/or indirect-acting aromatic amines in river water. Further study was made to identify 1-nitropyrene in XAD-2 resin extract from municipal river water.

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