INDUCED MICROSOMAL OXIDATION OF DICLOFOP, TRIASULFURON, CHLORSULFURON, AND LINURON IN WHEAT

Microsomal fractions from shoot tissues of etiolated wheat seedlings catalyzed the oxidation of diclofop, chlorsulfuron, triasulfuron, chlortoluron, and linuron. Microsomal oxidation products of chlorsulfuron, triasulfuron, and linuron were isolated and identified by mass spectrometry and cochromatography with reference standards. Oxidation was dependent on NADPH and molecular oxygen and was inhibited by CO in the presence of oxygen. Triasulfuron hydroxylation was inhibited to varying degrees by other known inhibitors of cytochrome P-450 enzymes and by several different postemergence herbicides. Enzyme activity was increased 2- to 3-fold by the removal of endogenous inhibitors and stimulated an additional 5- to 20-fold by the treatment of germinating seedlings with naphthalic anhydride, ethanol, or phenobarbital. In contrast to marked increases in monooxygenase activities following induction, microsomal cytochrome P-450 levels and NADPH cytochrome c reductase activities were not increased to a significant extent. Ethanol and phenobarbital were more effective than naphthalic anhydride as inducers of microsomal hydroxylase activity. The combined effect of naphthalic anhydride and ethanol as inducers of diclofop and triasulfuron hydroxylases was additive. Apparent Km values for triasulfuron, chlorsulfuron, and diclofop with constitutive and induced microsomal hydroxylases were compared. Differences in the response of herbicide monooxygenases to selected inhibitors, inducers, and substrates support the hypothesis that wheat microsomes contain a number of distinct cytochrome P-450-dependent monooxygenases with different substrate specificities and kinetic properties. These enzymes serve as important factors in the tolerance and selectivity of a broad spectrum of herbicides used in wheat production systems. © 1991.