Edith Ebert

Hydroxylation of primisulfuron by an inducible cytochrome P450-dependent monooxygenase system from maize

Abstract Microsomes were prepared from etiolated maize seedlings and incubated with [14C]primisulfuron (2-[3-(4,6-bis(difluoromethoxy)-pyrimidin-2-yl)-ureidosulfonyl]-benzoic acid methylester). Two enzymatic reaction products were formed in the presence of O2 and NADPH. Comparison on high-performance liquid chromatography with synthetic reference standards and mass spectrometry of the two in vitro metabolites revealed that [14C]primisulfuron was hydroxylated at two different sites, i.e., at the phenyl and at the pyrimidine ring, respectively. Both hydroxylation reactions were inhibited in vitro by tetcyclacis as well as by CO in the presence of O2. CO inhibition was reversed by irradiation of the reaction mixture with white light. Enzyme activities were localized predominantly in the shoots. Apparent Km values for [14C]primisulfuron were estimated to be 137 and 47 μM, and Vmax to be 427 and 261 pmol/hr/mg protein, for the hydroxylation on the pyrimidine and phenyl ring, respectively. Formation of these metabolites from [14C]primisulfuron was barely detectable in microsomes from germinating seedlings. However, seed treatment (0.2% w w ) with the safener, CGA 154281 (4-[dichloroacetyl]-3,4-dihydro-3-methyl-2H-1,4-benzoxazine), increased microsomal cytochrome P450 levels twofold and dramatically stimulated in vitro [14C]primisulfuron phenyl- and pyrimidine-ring hydroxylation. From these data it is concluded that hydroxylation of primisulfuron in maize microsomes is catalyzed by an inducible cytochrome P450 monooxygenase system.

Metabolism of the Aryloxyphenoxypropanoate Herbicide, CGA 184927, in Wheat, Barley and Maize: Differential Effects of the Safener, CGA 185072

Abstract The influence of the safener, CGA 185072 (5-chloro-8-quinolinoxy-acetic acid-1-methylhexyl-ester), on the metabolism of the aryloxyphenoxypropanoate herbicide, CGA 184927 (2-propynyl-R-2-[4-(5-chloro-3-fluoro-2-pyridinyloxy)-phenoxy]-propionate), was studied in excised leaves of wheat, barley, and maize. In wheat and barley, CGA 184927 readily underwent ester hydrolysis followed by hydroxylation at the pyridinyl moiety as well as ether cleavage between the pyridinyl and the phenyl ring. Ether cleavage constituted the minor pathway in both species. All metabolites were subject to glycosyl conjugation. Tetcyclacis strongly inhibited pyridinyl-ring hydroxylation in wheat. Metabolism by hydroxylation and ether cleavage was more rapid in wheat than in barley, and was found to be accelerated in the presence of the safener CGA 185072 in both wheat and, to a lesser degree, in barley. Moreover, the safener increased the capacity for O-glycoside formation in wheat as suggested from studies using the ,4C-labelled pyridinyl-ring hydroxylated metabolite as a precursor. In maize, which is highly susceptible to CGA 184927, rapid ester hydrolysis of CGA 184927 and partial conversion of the corresponding carboxylic acid to glycosyl ester conjugate(s) occurred. However, no further transformation of the herbicide was found in maize, both in the absence or presence of CGA 185072. It is concluded that the ability of CGA 185072 to protect wheat from injury by the herbicide, CGA 184927, and to confer partial protection to barley, is related to the ability of the safener to stimulate herbicide metabolism in these crop species.

A novel imidazole carboxylic acid ester is a herbicide inhibiting 14α-methyl-demethylation in plant sterol biosynthesis

Abstract CGA 201029/R 69020 [methyl 1-(2,2-dimethylindan-1-yl)imidazole-5-carboxylate], a novel imidazole carbonic acid ester, inhibits sterol biosynthesis in plants. Changes in sterol pattern were observed in maize roots exposed to 10−8 M of the inhibitor for 7 days. The compound lowered contents of stigma-, campe-, and sitosterol, while obtusifoliol and 14α-methylergast-8-en-3β-ol accumulated. Label from [14C]mevalonic acid mainly accumulated in obtusifoliol when tobacco cell suspension cultures were exposed to CGA 201029/R 69020, while label was predominantly found in 4-demethylsterols of controls. The compound interacted with a microsomal cytochrome P450 as shown by binding spectra studies. These data suggest that this substance inhibits obtusifoliol 14-methyl demethylase in plants.