Asulam (methyl N-(4-aminophenyl) sulfonylcarbamate) is a colorless crystalline solid used as a herbicide to control grasses and weeds on sugarcane and non croplands. This herbicide is mainly formulated and applied as asulam sodium salt. Asulam is 0.5% soluble in water while its salt is two hundred times more soluble. The half life of asulam in topsoil is up to 6 weeks (Balba et al., 1979).
According to the Environmental Protection Agency (EPA Asulam Fact Sheet) and some scientific evidence (Hu et al., 1992), asulam is considered a low toxic herbicide. Human exposure to asulam is mainly through diet. The Code of Federal Regulations (40 CFR 180.360) established a tolerance level of 0.1 ppm of asulam in sugarcane, but no additional information is reported for other food products. According to the EPA, asulam and its metabolites cannot exceed 272 ppb in surface water.
Aerobic degradation of asulam has been identified in Flavobacterium sp. (Walker, 1978), Curtobacterium sp. and Pseudomonas sp. (Balba et al., 1979). According to Balba, asulam is first transformed to sulfanilamide and then 4-aminobenzenesulfonate by two hydrolytic reactions. 4-Aminobenzenesulfonate is converted to 4-hydroxybenzenesulfonate and subsequent 1,2,4-benzenetriol by two oxidation reactions catalyzed by a monooxygenase and dioxygenase, respectively. 4-Aminobenzenesulfonate is also be converted to 4-Sulfocatechol via a deaminating dioxygenase. Intradiol cleavage of 4-sulfocatechol, lactonization, and subsequent hydrolysis forms maleylacetate (Blumel et al., 1998).
The following is a text-format asulam degradation pathway map. Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions. This map is also available in graphic (13k) format.
Asulam Flavobacterium sp. Curtobacterium sp. Pseudomonas sp. | | | asulam | hydrolase | v Sulfanilamide from the | 4-Carboxy- | 4'-sulfoazobenzene | sulfanilamide Pathway | hydrolase | | | v +----> 4-Aminobenzenesulfonate -------------------------+ | | | | 4-aminobenzenesulfonate | | 4-aminobenzenesulfonate 4-monooxygenase | | 3,4-dioxygenase | 4-sulfophenol | v monooxygenase v +---------> 4-Sulfophenol ---------------------> 4-Sulfocatechol | | | | | | from the | 4-hydroxybenzenesulfonate | protocatechuate 3-(4-Sulfophenyl)- | 1,2-dioxygenase | 3,4 dioxygenase typeII butyrate Pathway | | v v 1,2,4-Benzenetriol 3-Sulfomuconate | | | | | | 3-carboxymuconate v | cycloisomerase typeII to the | 2,4,5-Trichlorophenoxyacetic Acid v Pathway 4-Sulfolactone | | | sulfolactone hydrolase | | v Maleylacetate | | | v to the Chlorobenzene Pathway
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