Evidence for the stability and cytochrome P450 specificity of the phenobarbital-induced reductive halothane-cytochrome P450 complex formed in rat hepatic microsomes.
The hypothesis that the reduced spectral halothane-cytochrome P450 complex formed in rat hepatic microsomes is a stable cytochrome P450 specific species was examined. Comparisons of the cytochrome P450 inducers, phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN) and beta-naphthoflavone (beta-NF) showed that PB was the most effective inducer of the halothane-cytochrome P450 complex and the cytochrome P450 which liberates the halothane metabolites, 2-chloro-1,1-difluoroethene (CDE) and 2-chloro-1,1,1-trifluoroethane (CTE). However, the ratio of CDE produced to quantity of complex was found to be reduced 70-77% in these microsomes. A large portion of total microsomal cytochrome P450 was destroyed upon halothane reduction (up to 39%), yet the complexed cytochrome P450, particularly in microsomes from PB-treated animals, was resistant to the irreversible inactivation mechanisms of halothane reduction. The effects of reductive halothane metabolism on subsequent warfarin metabolism showed that 7-hydroxywarfarin formation from either (R)- or (S)-warfarin in microsomes from PCN-treated, PB-treated or untreated rats was highly susceptible to irreversible inhibition. In microsomes from PB-treated, but not PCN or untreated rats, the formation of one warfarin metabolite, 4'-hydroxywarfarin from (R)-warfarin, could be shown to be increased when complex was eliminated by photodissociation. These results suggest that PB-B is preferentially bound as complex and resistant to inactivation because of complex stability, and that halothane reduction readily destroys the cytochrome P450 form, PB-C.
Baker
M T
MT
Department of Anesthesia, University of Iowa, Iowa City 52242.
Vasquez
M T
MT
Chiang
C K
CK
D000818:Animals; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006221:Halothane / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D010084:Oxidation-Reduction; D010634:Phenobarbital / Q000494:pharmacology*; D011285:Pregnenolone Carbonitrile / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D014859:Warfarin / Q000378:metabolism
D065607:Cytochrome P-450 Enzyme Inhibitors; D011285:Pregnenolone Carbonitrile; D014859:Warfarin; D003577:Cytochrome P-450 Enzyme System; D006221:Halothane; D010634:Phenobarbital