Cytochrome P-450 inactivation by 3-alkylsydnones. Mechanistic implications of N-alkyl and N-alkenyl heme adduct formation.
Incubation of 3-(2-phenylethyl)-4-methylsydnone (PMS) with liver microsomes from phenobarbital-pretreated rats or with reconstituted cytochrome P-450b results in loss of the enzyme chromophore. Chromophore loss is NADPH-dependent even though the sydnone decomposes by an oxygen- but not enzyme-dependent process to give pyruvic acid and, presumably, the (2-phenylethyl)diazonium cation. N-(2-Phenylethyl)protoporphyrin IX and N-(2-phenylethenyl)protoporphyrin IX have been isolated from the livers of rats treated with PMS. Both deuteriums are retained in the N-(2-phenylethyl) adduct derived from 3-(2-phenyl[1,1-2H]ethyl)-4-methylsydnone, but one deuterium is lost in the N-(2-phenylethenyl) adduct. The N-(2-phenylethyl) to N-(2-phenylethenyl) adduct ratio is increased by deuterium substitution. No spectroscopically detectable intermediates precede chromophore loss in incubations of reconstituted cytochrome P-450b with PMS. Electron paramagnetic resonance (EPR)-spin trapping studies show that carbon radicals are formed in incubations of the sydnones with liver microsomes but by a process that is independent of chromophore destruction. It is proposed that the 2-phenylethyl radical formed by electron transfer to the sydnone-derived (2-phenylethyl)diazonium cation adds to the prosthetic heme group to give the N-(2-phenylethyl) adduct. This alkylation reaction is similar to that observed with (2-phenylethyl)hydrazine. Autoxidation of the Fe-CH(CH2Ph)-N bridged species expected from insertion of 2-phenyldiazoethane into one of the heme Fe-N bonds is proposed to explain the unprecedented introduction of a double bond into the N-(2-phenylethenyl) adduct.
Grab
L A
LA
Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco 94143.
Swanson
B A
BA
Ortiz de Montellano
P R
PR
D000818:Animals; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000302:isolation & purification; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme / Q000378:metabolism*; D007202:Indicators and Reagents; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D010069:Oxadiazoles / Q000494:pharmacology*; D010084:Oxidation-Reduction; D011485:Protein Binding; D011524:Protoporphyrins / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013558:Sydnones / Q000138:chemical synthesis / Q000494:pharmacology*
D065607:Cytochrome P-450 Enzyme Inhibitors; D007202:Indicators and Reagents; D010069:Oxadiazoles; D011524:Protoporphyrins; D013558:Sydnones; C057213:3-(2-phenylethyl)-4-methylsydnone; D006418:Heme; D009249:NADP; D003577:Cytochrome P-450 Enzyme System; C028025:protoporphyrin IX