Microsomal formation of nitric oxide and cyanamides from non-physiologicalN-hydroxyguanidines: N-hydroxydebrisoquine as a model substrate

The microsomal oxidative transformation of a non-physiological N-hydroxygua nidine was demonstrated for the first time for N-hydroxydebrisoquine as a m odel substrate (Clement et al., Biochem Pharmacol 46: 2249-2267, 1993). The objective of the present work was to further compare this reaction with th e analogous oxidation of arginine via N-hydroxyarginine to citrulline and n itric oxide. The oxidation of N-hydroxydebrisoquine by liver microsomes fro m rats pretreated with dexamethasone not only produced nitric oxide and the urea, but also the cyanamide derivative as the main metabolite. The low st ability of the cyanamide derivative, which easily hydrolyzed to the urea de rivative, was noted. The formation of all compounds required cosubstrate an d the enzyme source. Experiments with catalase, superoxide dismutase, and H 2O2 showed that the O-2(-) formed from the enzyme and the substrate apparen tly participated in the reaction. While the N-hydroxylation of the guanidin e involves the usual monooxygenase activity of cytochrome P-450 (Clement et al., Biochem Pharmacol 46: 2249-2267, 1993), the resultant N-hydroxyguanid ine decoupled the monooxygenase. Nitric oxide was detected by the oxyhemogl obin assay. To examine the influence of enzymatically formed nitric oxide o n the formation of the metabolites, the N-hydroxydebrisoquine was incubated with SIN-1 as nitric oxide donor under aerobic conditions. It was again po ssible to detect the cyanamide and urea derivatives, with the latter as mai n metabolite. It was concluded that the microsomal transformation of N-hydr oxydebrisoquine produces a cyanamide and nitric oxide which reacts with N-h ydroxydebrisoquine to form the urea derivative. The purely chemical reactio n of the unsubstituted N-hydroxyguanidine with nitric oxide gave similar re sults (Fukuto et al., Biochem Pharmacol 43: 607-613, 1992). In conclusion, similarities (formation of a urea derivative) and differences (formation of a cyanamide derivative) between the physiological oxidation of N-hydroxy-L -arginine by nitric oxide synthases and non-physiological N-hydroxyguanidin es by cytochrome P-450 were observed. Furthermore, non-physiological N-hydr oxyguanidines can be regarded as nitric oxide donors. (C) 1999 Elsevier Sci ence Inc.