Modulation of the reductive metabolism of halothane by microsomal cytochrome b5 in rat liver.

To study the modulation of the reductive metabolism of halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) by microsomal cytochrome b5, formation of 2-chloro-1,1,1-trifluoroethane (CTE) and 2-chloro-1,1-difluoroethylene (CDE), major reduced metabolites of halothane, was analyzed in vivo and in vitro. Rats were pretreated with both malotilate (diisopropyl-1,3-dithiol-2-ylidenemalonate) and sodium phenobarbital (malotilate-treated rats) or only with sodium phenobarbital (control rats). The microsomes of malotilate-treated rats had significantly more cytochrome b5 than the controls, whereas the cytochrome P-450 content was not different between the two groups. At the end of 2-h exposure to 1% halothane in 14% oxygen, the ratio of CDE to CTE in arterial blood was significantly higher in malotilate-treated rats than in the controls. Under anaerobic conditions, the formation of CDE and the ratio of CDE to CTE were significantly greater in microsomal preparations of malotilate-treated rats than those of the controls. In a reconstituted system containing cytochrome P-450PB purified from rabbit liver, addition of cytochrome b5 to the system enhanced the formation of CDE and increased the ratio of CDE to CTE. These results suggested that cytochrome b5 enhances the formation ratio of CDE to CTE by stimulating the supply of a second electron to cytochrome P-450, which might reduce radical reactions in the reductive metabolism of halothane.