Differential Metabolism of the Sulfonylurea Herbicide Prosulfuron (CGA-152005) by Plant Microsomes

Microsomes isolated from excised shoots of 3-day-old. dark grown, grain sorghum [Sorghum bicolor (L.) Moench, Funk G522DR and DK 41Y] and corn seedlings [Zea mays (L.), Pioneer 3245] metabolized the sulfonylurea herbicide prosulfuron (CGA-152005). Corn microsomes predominantly formed a single major metabolite that resulted from hydroxylation of the phenyl ring at the C5 position. However, sorghum microsomes formed two major metabolites in an approximate 1:1 ratio. One was the 5-hydroxyphenyl metabolite, whereas the second metabolite resulted from ö-demethylation at C4 of the triazine ring. Metabolite identity was established by mass spectrometry and co-chromatography with authentic standards. Metabolism in both corn and sorghum was greatly enhanced by pretreatment of the seed with naphthalic anhydride and by subirrigation with 2.5% ethanol 24 h prior to harvest. Metabolism required a reduced pyridine nucleotide and was affected by several cytochrome P450 monooxygenase inhibitors (carbon monoxide, tetcyclacis, piperonyl butoxide, 1 aminobenzotriazole, and SKF-525A). The inhibitors differentially affected metabolism of prosulfuron. Microsomal oxidations from both untreated and inducer-treated tissue responded similarly to the inhibitors. In exploratory studies, microsomes isolated from shoots of wheat [Triticum aestivum L., Pioneer 2548], barley [Hordeum vulgare L., Boone], oats [Avena sativa L., Southern States 76-30-P242] and rice [Oryza sativa L„ Gulfmont], and room ripened avocado [Persea americana, Mill., Hass] mesocarp tissue also primarily formed the 5-hydroxyphenyl metabolite. Titration of seven different avocado microsomal preparations with prosulfuron provided typical type I difference spectra from which an average binding constant (K ) of 187 ± 35 μm was obtained