ISOFLURANE AND CYTOCHROME B(5) STIMULATION OF 2-CHLORO-1,1-DIFLUOROETHENE METABOLISM BY RECONSTITUTED RAT CYP2B1 AND CYP2C6

Isoflurane stimulates the metabolism of 2-chloro-1,1-difluoroethene (C DE) in liver microsomes from phenobarbital-treated rats or rabbits. Th e P450 isozymes involved and the mechanism by which such stimulation o ccurs have not been clarified. The present study examined the effects of isoflurane and cytochrome b(5) on CDE metabolism in reconstituted s ystems containing purified rat CYP2B1 or CYP2C6. Under similar incubat ion conditions, CYP2B1 defluorinated CDE at approximately five times t he rate of CYP2C6. Isoflurane was a potent stimulator of CDE metabolis m, increasing it nearly 5-fold when catalyzed by CYP2B1, but only 2-fo ld when catalyzed by CYP2C6. Isoflurane had no stimulatory effect on b enzphetamine metabolism by CYP2B1 or CYP2C6. Cytochrome b(5) was not r equired for isoflurane-facilitated CDE metabolism; however, the additi on of cytochrome b(5) to CYP2B1 increased CDE metabolism 71 and 44%, i n the absence and presence of isoflurane, respectively. In reconstitut ed CYP2B1, isoflurane generated a type I difference spectrum of approx imately twice the magnitude of CDE and stimulated NADPH consumption mo re so than CDE. The same quantity of NADPH was consumed when CDE was p resent with isoflurane as compared with isoflurane alone. These data s upport the hypothesis that isoflurane stimulates CDE metabolism by a m echanism involving increased P450 reduction via direct isoflurane inte raction with P450.