Efficient formation of nitric oxide from selective oxidation of N-aryl N '-hydroxyguanidines by inducible nitric oxide synthase

Inducible nitric oxide synthase (NOS II) efficiently catalyzes the oxidatio n of N-(4-chlorophenyl)N'-hydroxyguanidine 1 by NADPH and O-2, With concomi tant formation of the corresponding urea and NO. The characteristics of thi s reaction are very similar to those of the NOS-dependent oxidation of endo genous N-omega-hydroxy-L-arginine (NOHA), i.e., (i) the formation of produc ts resulting from an oxidation of the substrate C=N(OH) bond, the correspon ding urea and NO, in a 1:1 molar ratio, (ii) the absolute requirement of th e tetrahydrobiopterin (BH4) cofactor for NO formation, and (iii) the strong inhibitory effects of L-arginine (L-arg) and classical inhibitors of NOSs. N-Hydroxyguanidine 1 is not as good a substrate for NOS II as is NOHA (K-m = 500 mu M versus 15 mu M for NOHA). However, it leads to relatively high rates of NO formation which are only 4-fold lower than those obtained with NOHA (V-m = 390 +/- 50 nmol NO min(-1) mg protein(-1), corresponding roughl y to 100 turnovers min(-1)). Preliminary results indicate that some other N -aryl N'-hydroxyguanidines exhibit a similar behavior. These results show f or the first time that simple exogenous compounds may act as NO donors afte r oxidative activation by NOSs. They also suggest a possible implication of NOSs in the oxidative metabolism of certain classes of xenobiotics.