Inhibition of Acetolactate Synthase Isozyme II fromEscherichia coliby a New Azido-Photoaffinity Sulfonylurea

The sulfonylurea herbicides are very potent inhibitors of acetolactate synthase (ALS). These compounds have been reported as “extraneous inhibitors” due to the fact that their inhibition site corresponds to neither the catalytic site nor the regulatory sites of the enzyme. So far, the complexity of the ALS reaction and the reversible binding mode of sulfonylureas have hampered any attempt to locate the inhibitor domain. Toward this goal, a photoactivatable azidosulfonylurea has been synthesized. The azido derivative was analyzed for its photo- chemical andin vitrobiological properties toward the bacterial ALS isozyme II. Similar to other ALS inhibitors, azidosulfonylurea potently inhibited ALS II with estimated initial and final dissociationKiconstant values of 52 and 300 nM, respectively, and slowly inactivated the enzyme. After inhibition, removal of the free azido inhibitor and precipitation with ammonium sulfate of the azidosulfonylurea/ALS II complex led to complete though slow recovery of the enzyme activity. Following photoreaction of the inhibited complex and removal of the free inhibitor under the same conditions, the bacterial enzyme conversely exhibited stable inactivation. These results suggest that the newly synthesized azidosulfonylurea is capable of undergoing covalent reaction with ALS II, and hence it might be useful, once radiolabeled, to shed light on the inhibitor binding site of ALS.