Metabolism of the Herbicide Isoproturon in Wheat and Soybean Cell Suspension Cultures

Abstract

The plant metabolism of the phenylurea herbicide isoproturon (IPU) was studied in aseptic cell suspension cultures of soybean and wheat. After incubation with the14C-labeled herbicide (1 ppm) over 48 h, total metabolic rates were 45–54%. The predominant metabolites in soybean were identified as monodesmethyl-IPU, 2-hydroxy-IPU, and 2-hydroxy-monodesmethyl-IPU. The major metabolic pathway identified for wheat was the conversion of IPU to 2-hydroxy-IPU as the primary metabolite and then to 2-hydroxy-monodesmethyl-IPU. 2-Hydroxy-IPU and an olefinic metabolite (isopropenyl-IPU) are described here for the first time. Metabolite identification was based on high-performance liquid chromatography retention times as well as time-of-flight/secondary-ion mass spectrometry and1H nuclear magnetic resonance spectroscopy. Wheat microsomes catalyzed the NADPH-dependent conversion of isoproturon to monodesmethyl and 2-hydroxy-monodesmethyl metabolites. 2-Hydroxy-IPU was only a minor enzymatic product so there was a remarkable difference between the primary cellular and enzymatic metabolites.14C Incorporation into the wheat-insoluble residue fraction increased from 3% (48 h) to 14% (7 days). A sequential solubilization procedure indicated that binding occurred mainly to the operationally defined hemicellulose (65%) and lignin (10%) fractions. Simulated stomach conditions (pH 1, 37° C, 24 h) released only about 5% of total bound radioactivity. The similarity of IPU metabolism in microbial, plant, and animal systems is discussed, with 2-hydroxy-monodesmethyl-IPU being a common metabolite.