INTERACTION OF THE HERBICIDE SULFOMETURON METHYL WITH ACETOLACTATE SYNTHASE: A SLOW-BINDING INHIBITOR John V. Schloss Central Research & Development Department E. I. du Pont de Nemours & Company Wilmington, Delaware 19898 USA Sulfometuron methyl (SM) (Fig. 1), the active component of the ® commercial herbicide Oust , is a potent and selective inhibi- tor of acetolactate synthase (ALS) (E.C. 4.1.3.18) in various bacteria (1,2), yeast (3), and plants (4,5). ALS is the first common enzyme in the biosynthesis of the branched-chain amino acids. This enzyme has an absolute requirement for FAD (1,6,7) although an ALS involved in the biosynthesis of acetoin from Aerobacter aerogenes does not require FAD (8). Since the reaction catalyzed by ALS involves no net redox, the require- ment for FAD is unusual. Spectral changes in ALS-bound FAD upon addition of SM, or upon initiating the enzymic reaction, together with the kinetic details of ALS's inhibition by SM are reported here. Results and Discussion Inhibition of salmonella typhimurium acetolactate synthase isozyme II (ALSII) by SM was biphasic. Figure 2 illustrates assay progress curves (obtained by continuously monitoring pyruvate consumption at 333 nm, e = 17.5 cm * M in the presence of various concentrations of SM. From the data in Fig. 2, an initial K^ of 1.7 yM, a final steady-state K^ of 82 nM, and a maximal rate constant for transition between initial and final inhibition of 0.15 min ^ were obtained, values that are in reasonable agreement with those previously Flavins and Flavoproteins © 1984 Walter de Gruyter & Co., Berlin  New York - Printed in Germany