INTERFERON-GAMMA-INDUCIBLE MURINE MACROPHAGE NITRIC-OXIDE SYNTHASE - STUDIES ON THE MECHANISM OF INHIBITION BY IMIDAZOLE AGENTS

Citrulline formation by the interferon-gamma/lipopolysaccharide-inducible murine macrophage nitric oxide synthase is inhibited reversibly by imidazole, 1-phenylimidazole, 4-phenylimidazole, and 2-phenylimidazole with IC50 values of 40 mu M, 6 mu M, 225 mu M and >1 mM, respectively. 1-Phenylimidazole inhibited the maximal velocity of citrulline formation but did not alter the concentration of arginine providing half-maximal activity. 1-Phenylimidazole inhibited citrulline formation by the murine macrophage nitric oxide synthase competitively versus (6R)-5,6,7,8-tetrahydro-L-biopterin (THB) with a K-i value of 0.7 mu M, but inhibited citrulline formation by Ca2+-calmodulin-dependent nitric oxide synthase from GH(3) pituitary cells noncompetitively versus THB with a K-i value of 40 mu M. Imidazole inhibited citrulline formation by the murine macrophage nitric oxide synthase noncompetitively versus THB with a K-i value of 48 mu M. Neither imidazole nor 1-phenylimidazoIe inhibited the cytochrome c reductase activity of murine macrophage nitric oxide synthase at concentrations 100-fold higher than their IC50 values for inhibiting citrulline formation. The antifungal imidazoles miconazole, ketoconazole, and clotrimazole did not inhibit either citrulline formation or cytochrome c reduction by murine macrophage nitric oxide synthase at concentration as high as 200 mu M. Ca2+-calmodulin-dependent nitric oxide synthase from GH(B) pituitary cells exhibited a K-act for THB of 80 nM, while the inducible murine macrophage nitric oxide synthase exhibited a K-act of 8 mu M. (C) 1994 Academic Press, Inc.