Microsomal cytochrome P450 dependent oxidation of N-hydroxyguanidines, amidoximes, and ketoximes: Mechanism of the oxidative cleavage of their C=N(OH) bond with formation of nitrogen oxides

Oxidation by rat liver microsomes of 13 compounds involving a C=N(OH) funct ion (including N-hydroxyguanidines, amidoximes, ketoximes, and aldoximes) w as found to occur with the release of nitrogen oxides such as NO, NO2-, and NO3-. The greatest activities were observed with liver microsomes from dex amethasone-treated rats (up to 8 nmol of NO2- nmol of P450(-1) min(-1)). A detailed study of the microsomal oxidation of some of these compounds was p erformed. Oxidation of N-(4-chlorophenyl)-N'-hydroxy guanidine led to the f ormation of the corresponding urea and cyanamide in addition to NO, NO2-, a nd NO3-. Formation of all these products was dependent on NADPH, Oz, and cy tochromes P450. Oxidation of two arylamidoximes was found to occur with for mation of the corresponding amides and nitriles in addition to nitrogen oxi des. Oxidation of 4-(chlorophenyl)methyl ketone oxime gave the correspondin g ketone and nitroalkane as well as NO, NO2-, and NO3-. These reactions wer e also dependent on cytochromes P450 and required NADPH and Oz. Mechanistic experiments showed that microsomal oxidations of amidoximes to the corresp onding nitriles and of ketoximes to the corresponding nitroalkanes are not inhibited by superoxide dismutase (SOD) and are performed by a cytochrome P 450 active species, presumably the high-valent P450-iron-oxo complex. On th e contrary, microsomal oxidation of N-hydroxyguanidines to the correspondin g cyanamides was greatly inhibited by SOD and appeared to be mainly due to O-2(.-) derived from the oxidase function of cytochromes P450, Similarly, m icrosomal oxidations of N-hydroxyguanidines and amidoximes to the correspon ding ureas and amides were also found to be mainly performed by O-2(.-), as shown by the great inhibitory effect of SOD (70-100%) and the ability of t he xanthine-xanthine oxidase system to give similar oxidation products. How ever, it is noteworthy that other species, such as the P450 Fe(II)-O-2 comp lex, are also involved, to a minor extent, in the SOD-insensitive microsoma l oxidative cleavages of compounds containing a C=N(OH) bond. Our results s uggest a general mechanism for such oxidative cleavages of C=N(OH)bonds wit h formation of nitrogen oxides by cytochromes P450 and NO-synthases, with t he involvement of O-2(.-) and its Fe(III) complex [(FeIII-O-2(-)) or (FeII- O-2)] as main active species.