Oxygen Concentrations Required for Reductive Defluorination of Halothane by Rat Hepatic Microsomes

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Abstract
The free oxygen concentrations required for reductive defluorination of halothane by rat hepatic microsomes from control and phenobarbital- (PB) and polychlorinated biphenyl- (PCB) treated animals were determined. Halothane-exposed microsomes from treated rats generated measurable levels of fluoride ion after 30 min incubations with oxygen concentrations of 5% or less. Microsomes from control animals produced fluoride only if the free oxygen concentration was 2% or less. During anoxic (0% oxygen) incubations, defluorination rates of 2.10 ± 0.17, 5.55 ± 0.38, and 5.46 ± 0.30 nmol fluoride·mg protein −1·30 min−1 were observed for microsomes from control, PB, and PCB rats, respectively. Normalizing the maximal rates of defluorination to the microsomal cytochrome P-450 content yielded values of 5.14 ± 1.40, 3.70 ± 0.15, and 2.38 ± 0.26 nmol fluoride·nmol cytochrome P-450−1·30 min−1 for control, PB, and PCB microsomes, respectively. Oxygen concentrations required for reductive metabolism of halothane by isolated rat hepatic microsomes are close to normal physiologic free oxygen concentrations of 4–5% reported for centrilobular areas of the rat liver. Thus even slight reductions in hepatic oxygenation during anesthetic exposure could lead to enhanced reductive biotransformation, an observation found in rat models of halothane-associated hepatic injury. The large differences among the treatment groups in the rates of fluoride ion generated per nanomole cytochrome P-450 indicate that enzyme induction regimens disproportionately increase those isozymes of hepatic cytochrome P-450 that are not involved with the reductive defluorination of halothane.