Conformationally-restricted arginine analogues as alternative substrates and inhibitors of nitric oxide synthases Original Research Article

Conformationally restricted arginine analogues (1–5) were synthesized and found to be alternative substrates or inhibitors of the three isozymes of nitric oxide synthase (NOS). A comparison of kcat/Km values shows that (E)-3,4-didehydro-d,l-arginine (1) is a much better substrate than the corresponding (Z)-isomer (2) and 3-guanidino-d,l-phenylglycine (3), although none is as good a substrate as is arginine; 5-keto-d,l-arginine (4) is not a substrate, but is an inhibitor of the three isozymes. Therefore, it appears that arginine binds to all of the NOS isozymes in an extended (E-like) conformation. None of the compounds exhibits time-dependent inhibition of NOS, but they are competitive reversible inhibitors. Based on the earlier report that Nω-propyl-l-arginine is a highly selective nNOS inhibitor (Zhang, H. Q.; Fast, W.; Marletta, M.; Martasek, P.; Silverman, R. B. J. Med. Chem. 1997, 40, 3869), (E)-Nω-propyl-3,4-didehydro-d,l-arginine (5) was synthesized, but it was shown to be weakly potent and only a mildly selective inhibitor of NOS. Imposing conformational rigidity on an arginine backbone does not appear to be a favorable approach for selective NOS inhibition.