CALMODULIN-DEPENDENT NITRIC-OXIDE SYNTHASE - MECHANISM OF INHIBITION BY IMIDAZOLE AND PHENYLIMIDAZOLES

Calmodulin-dependent nitric-oxide synthase from bovine brain and GH3 P ituitary cells is inhibited by imidazole, 1-phenylimidazole, 2-phenyli midazole, and 4-phenylimidazole, with half-maximal inhibition occurrin g at 200, 25, 160, and 600 muM concentrations of inhibitor, respective ly. Imidazole inhibits the maximal velocity of citrulline formation by the enzyme, but does not alter the concentration of arginine, calmodu lin, or (6R)-5,6,7,8,-tetrahydro-L-biopterin required for expression o f half-maximal activity. Imidazole, 1-phenylimidazole, 2-phenylimidazo le, and 4-phenylimidazole had no effect on calmodulin-dependent reduct ion of cytochrome c by the enzyme at concentrations up to 50-fold high er than those that inhibited citrulline formation. Imidazole inhibited calmodulin-dependent NADPH consumption by the enzyme with dissolved o xygen as the sole electron acceptor, with half-maximal inhibition occu rring at a concentration of 225 muM. These observations are consistent with the proposal that imidazole and phenylimidazoles inhibit citrull ine formation and oxygen reduction by acting as a sixth coordination l igand of the heme iron. This interaction prevents the formation of the activated reduced species of oxygen necessary for the formation of ci trulline.