Enflurane and methoxyflurane: Their interaction with hepatic cytochrome P-450 in vitro

The binding and metabolism of enflurane (CClFHCF₂OCF₂H) and of methoxyflurane (CCl₂HCF₂OCH₃) were investigated in vitro with hepatic microsomes from male rats. The metabolism of these anesthetic agents was monitored by NADPH consumption and fluoride ion production. In addition, from methoxyflurane, the production of acid-labile fluoride was monitored. The effects of inducing agents for different forms of cytochrome P-450 on the binding (K_s, ΔA_max) and metabolism (K_M, V_max) of both anesthetic agents are reported. The effects of CO, SKF-525A and metyrapone on the production of fluoride from enflurane and on fluoride and acid-labile fluoride from methoxyflurane are compared. The results of these studies indicate that only a form of cytochrome P-450 induced by phenobarbital binds and metabolizes enflurane in vitro. In contrast, at least two forms of cytochrome P-450 are involved in the binding and metabolism of methoxyflurane in vitro. Methoxyflurane interacts with a form of cytochrome P-450 induced by phenobarbital and at least one other form of cytochrome P-450, but not with cytochrome P-448. The relative amounts of free and acid-labile fluoride produced from methoxyflurane in vitro are altered after induction with 3-methylcholanthrene but not with phenobarbital. Although K_s, ΔA_max, K_M and V_max (NADPH consumption) are similar in magnitude for enflurane and methoxyflurane, the rate of production of fluoride from enflurane is approximately 8-fold less than from methoxyflurane. The observed stoichiometry of 140:1 for NADPH consumption to fluoride production for enflurane suggests that enflurane is enhancing NADPH oxidation far in excess of its metabolism. The relevance of these results to the proposed pathways for the metabolism of enflurane and methoxyflurane is discussed. The results are compared with reports of the metabolism and toxicity of enflurane and methoxyflurane in vivo.