title: Role of superoxide in the N-oxidation of N-(2-methyl-1-phenyl-2-propyl)hydroxylamine by the rat liver cytochrome P-450 system.
abstract: The N-oxidation of N-(2-methyl-1-phenyl-2-propyl)hydroxylamine (N-hydroxyphentermine, MPPNHOH) and the N-hydroxylation of 2-methyl-1-phenyl-2-propylamine (phentermine) by reconstituted systems that contained cytochromes P-450 purified from rat liver microsomes were demonstrated. The oxidation of MPPNHOH, but not of phentermine, could also be mediated by a superoxide and hydrogen peroxide generating system that contained xanthine and xanthine oxidase. Superoxide dismutase completely inhibited the oxidation of MPPNHOH by the xanthine/xanthine oxidase system and inhibited by 70% the oxidation mediated by a reconstituted cytochrome P-450 oxidase system. The majority of the microsomal oxidation was inhibited by an antibody raised against the major isozyme of cytochrome P-450 purified from livers of phenobarbital-pretreated rats. 2-Methyl-2-nitroso-1-phenylpropane (MPPNO) was found to be an intermediate in the overall oxidation of MPPNHOH to 2-methyl-2-nitro-1-phenylpropane (MPPNO2). Superoxide dismutase appeared to inhibit the first step, the conversion of MPPNHOH to MPPNO. These observations are accounted for by a sequence of two mechanistically distinct P-450-mediated oxidations. In the first reaction, N-hydroxylation of phentermine occurs by a normal cytochrome P-450 pathway. The formed hydroxylamine then uncouples the cytochrome P-450 system to generate superoxide and hydrogen peroxide. The superoxide oxidizes MPPNHOH to MPPNO which is then oxidized to MPPNO2, the ultimate product. This superoxide-mediated oxidation represents another pathway for N-oxidation by cytochrome P-450.
mesh_terms: D000818:Animals; D000936:Antigen-Antibody Complex; D003577:Cytochrome P-450 Enzyme System / Q000302:isolation & purification / Q000378:metabolism*; D007074:Immunoglobulin G; D007700:Kinetics; D008862:Microsomes, Liver / Q000378:metabolism*; D009251:NADPH-Ferrihemoprotein Reductase / Q000302:isolation & purification / Q000378:metabolism; D010084:Oxidation-Reduction; D010645:Phentermine / Q000031:analogs & derivatives* / Q000378:metabolism; D051381:Rats; D013482:Superoxide Dismutase / Q000378:metabolism; D013481:Superoxides / Q000378:metabolism*
chemical_list: D000936:Antigen-Antibody Complex; D007074:Immunoglobulin G; D013481:Superoxides; C013568:N-hydroxyphentermine; D003577:Cytochrome P-450 Enzyme System; D010645:Phentermine; D013482:Superoxide Dismutase; D009251:NADPH-Ferrihemoprotein Reductase