Deuterium isotope effects and product studies for the oxidation of N-omega-allyl-L-arginine and N-omega-allyl-N-omega-hydroxy-L-arginine by neuronal nitric oxide synthase

Abstract

The nitric oxide synthases (NOS), which require heme, tetrahydrobiopterin, FMN, FAD, and NADPH, catalyze the O-2-dependent conversion of L-arginine to L-citrulline and nitric oxide. N-omega-Allyl-L-arginine, a mechanism-based inactivator of neuronal NOS, also is a substrate, producing L-arginine, acrolein, and H2O (Zhang, H. Q., Dixon, R. P.; Marletta, R I. A.; Nikolic, D.; Van Breemen, R.; Silverman, R. B. J. Am. Chem. SOC. 1997, 119, 10888). Two possible mechanisms for this turnover an proposed, one initiated by allyl C-I-I bond cleavage and the other by guanidino N-H cleavage, and these mechanisms are investigated with the use of N omega-allyl-L-arginine (1), N-omega-[1,1-H-2(2)]allyl-L-arginine (7), N-omega-allyl-L-hydroxy-L-alginine (2) and N-omega-[1,1-H-2(2)]allyl-N-omega-hydroxy-L-arginine (8) as substrates. Significant isotope effects on the two kinetic parameters, k(cat) and k(cat)/k(m) are observed in case of 1 and 7 during turnover, but not with 2 and 8. No kinetic isotope effects are observed for either compound in their role as inactivators. These results support a mechanism involving initial CH bond cleavage of N-omega-allyl-L-arginine followed by hydroxylation and beakdown to products. (C) 2000 Published by Elsevier Science Ltd.