DIFFERENTIAL METABOLISM OF THE SULFONYLUREA HERBICIDE PROSULFURON (CGA-152005) BY PLANT MICROSOMES

Microsomes isolated from excised shoots of 3-day-old, dark grown, grai n sorghum [Sorghum bicolor (L.) Moench, Funk G522DR and DK 41Y] and co rn seedlings [Zea mays (L.), Pioneer 3245] metabolized the sulfonylure a herbicide prosulfuron (CGA-152005). Corn microsomes predominantly fo rmed a single major metabolite that resulted from hydroxylation of the phenyl ring at the C5 position. However, sorghum microsomes formed tw o major metabolites in an approximate 1:1 ratio. One was the 5-hydroxy phenyl metabolite, whereas the second metabolite resulted from O-demet hylation at C4 of the triazine ring. metabolite identity was establish ed by mass spectrometry and co-chromatography with authentic standards . Metabolism in both corn and sorghum was greatly enhanced by pretreat ment of the seed with naphthalic anhydride and by subirrigation with 2 .5% ethanol 24 h prior to harvest. Metabolism required a induced pyrid ine nucleotide and was affected by several cytochrome P450 monooxygena se inhibitors (carbon monoxide, tetcyclacis, piperonyl butoxide, 1 ami no-benzotriazole, and SKF-525A). The inhibitors differentially affecte d 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 [Triticu m aestivum L. Pioneer 2548], barley [Hordeum vulgare L., Boone], oats [Avena sativa L., Southern States 76-30-P242] and rife [Oryza sativa L ., Gulfmont], and room ripened avocado [Persen americana Mill., Hass] mesocarp tissue also primarily formed the 5 hydroxyphenyl metabolite. Titration of seven different avocado microsomal preparations with pros ulfuron provided typical type I difference spectra from which an avera ge binding constant (K-s) of 187 +/- 35 mu M was obtained.