Enflurane and methoxyflurane: Their interaction with hepatic microsomal stearate desaturase

Abstract

The effects of the volatile anesthetic agents enflurane (CClFHCF₂OCF₂H) and methoxyflurane (CCl₂HCF₂OCH₃) on hepatic microsomal electron transfer components and stearate desaturase are reported. Both enflurane and methoxyflurane stimulated electron now from NADH and NADPH through hepatic microsomal cytochrome b₅. The stimulation of electron flow from cytochrome b₅ by the anesthetic agents was not inhibited by metyrapone or CO, but was inhibited by 0.5 mM KCN. The effects of enflurane and methoxyflurane were influenced by the diet and pretreatment of the rat prior to death. A high-carbohydrate diet enhanced the effects, while fasting with or without phenobarbitone treatment diminished them. The anesthetic agents did not affect the rate constant for the autoxidation of purified trypsin-cleaved cytochrome b₅ or the activity of hepatic microsomal NADH- and NADPH-cytochrome c reductase, except that enflurane slightly increased the activity of NADH-cytochrome c reductase. The values of the equilibrium constants (K_eq) for the stimulation of the oxidation of hepatic microsomal cytochrome b₅ by enflurane and methoxyflurane were determined to be 1.2 and 0.5 mM, respectively. The K_eq for enflurane differed from the K_s and K_m values for the interaction of this anesthetic agent with cytochrome P-450, whereas the K_eq for methoxyflurane differed from the K_m for NADPH oxidation by cytochrome P-450, but not from the K_s for binding to cytochrome P-450 or the K_m for fiuoride ion production from this anesthetic agent by cytochrome P-450. The K_i values of 0.08 and 0.11 mM obtained for cyanide inhibition of the enhancement of the oxidation of cytochrome b₅ by enflurane and methoxyflurane, respectively, are within experimental error of the K_i for cyanide inhibition of stearate desaturase. Enflurane and methoxyflurane, however, did not inhibit the conversion of stearoyl CoA to oleate by hepatic microsomal stearate desaturase. It is concluded that enflurane and methoxyflurane stimulate hepatic microsomal electron flow from NADH and NADPH through cytochrome b₅in vitro, apparently by interacting with stearate desaturase.