Metabolism of the carbamate herbicide, asulam, in the rat.

The metabolism of [ring-14C]asulam, a systemic herbicide highly effective against bracken, has been studied in rats. Most of the radioactivity (76-100% dose) administered orally or intravenously is excreted in the urine in 24 h as unchanged asulam (61-74% dose), N4-acetylasulam (8-14%) and N4-acetylsulphanilamide (0.1-2.6%). Small amounts of radioactivity (0.3-7.4% dose) were present in the faeces, only traces (0.2-0.3%) were excreted in the bile, and no significant 14CO2 was detected. Perfusion of rat liver with 14C-asulam resulted in more extensive metabolism. Total amounts present in perfusate (81% total), bile (1%) plus liver (14%) were 23.1% for unchanged asulam, 25.7% for acetylasulam, less than 1% for acetylsulphanilamide, and 4.5% as conjugates of asulam and acetylasulam, together with several other unidentified metabolites. Asulam is acetylated more readily than sulphanilamide, by rat-liver homogenate, and the highest enzyme activity was associated with the mitochondrial fraction (2.4 pmol/mg protein per min). Although not hydroxylated by rats in vivo, evidence was obtained for the hydroxylation of asulam by rat-liver microsomal preparations in vitro.