Nω-propargyl-l-arginine and Nω-hydroxy-Nω-propargyl-l-arginine are inhibitors, but not inactivators, of neuronal and macrophage nitric oxide synthases Original Research Article

Nω-Propargyl-l-arginine (7) was synthesized as a potential mechanism-based inactivator of neuronal nitric oxide synthase (nNOS) and macrophage nitric oxide synthase (iNOS). Compound 7 is a potent reversible competitive inhibitor for both isoforms, having Ki values of 430 ± 50 nM and 620 ± 30 nM for nNOS and iNOS, respectively. These values are 12 and 32 times lower than the Km for l-arginine with nNOS and iNOS, respectively; however, 7 does not exhibit time-dependent inhibition with either. It also only undergoes oxidation very slowly. Nω-Hydroxy-Nω-propargyl-l-arginine also was synthesized to determine if the initial proposed enzyme-catalyzed hydroxylation of Nω-propargyl-l-arginine was problematic. This compound also is a potent reversible inhibitor of both nNOS and iNOS, but is not a time-dependent inactivator and is oxidized only very slowly. These results are in sharp contrast with the corresponding olefins, Nω-allyl-l-arginine and Nω-allyl-Nω-hydroxy-l-arginine recently reported to be potent time-dependent, irreversible inhibitors of nNOS (Zhang, H.Q.; Dixon, R.P.; Marletta, M.A.; Silverman, R.B., J. Am. Chem. Soc. 1997, 119, in press); Nω-allyl-l-arginine also was reported to be an inactivator of iNOS (Olken, N.M.; Marletta, M.A. J. Med. Chem. 1992, 35, 1137). This suggests that the active site of both isoforms of NOS can accommodate a variety of structures, but binding must have the appropriate juxtaposition for hydroxylation; otherwise, no oxidation occurs.